What Is the Plural of Diagnosis?

The correct plural of the word diagnosis is diagnoses. This means that when you are talking about more than one diagnosis, you do not add an extra “s” to make “diagnosises.”

Instead, the “is” at the end of the word changes to “es.” This form comes directly from its Greek origin and is preserved in modern English for accuracy and tradition.

The word diagnosis itself comes from the Greek word “διάγνωσις,” which means “a distinguishing” or “knowing apart.”

In Greek, nouns ending in “-is” usually form their plural by replacing the “-is” with “-es.” English borrowed this pattern and applies it to many formal and medical terms, including diagnosis.

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Why is it not “diagnosises”?

At first glance, you might think that since most English words take “s” or “es” to become plural, “diagnosises” would make sense. But this would go against the established rule for Greek-derived words ending in “-is.”

In standard English, particularly in medical and academic contexts, the correct plural is formed by changing “is” to “es.”

This same pattern appears in other words such as crisis which becomes crises, analysis which becomes analyses, and prognosis which becomes prognoses.

This is one of the many examples of irregular plurals in English. It does not follow the usual rule, but rather follows the historical form from the language it came from. Linguists and grammar experts have documented this pattern in multiple academic and medical writing style guides.

How common is this rule in medical terminology?

If you look through any medical dictionary, you will find this rule repeatedly. Words ending in “-is” in medical terms almost always form their plural by becoming “-es.”

This is why doctors, nurses, and medical researchers will write diagnoses instead of diagnosises in patient records or research reports.

Examples include thrombosis becoming thromboses, psychosis becoming psychoses, and neurosis becoming neuroses. This consistency makes it easier for medical professionals to communicate clearly without confusion.

Clear examples of singular and plural usage

To make the difference even more obvious, here are four distinct examples of how to use diagnosis and diagnoses in real sentences:

1. When speaking of one finding, you could say: “The diagnosis revealed a bacterial infection in her lungs.” In this case, only one condition has been identified.

2. When a patient has more than one identified condition, you might say: “Over the last five years, the patient received multiple diagnoses including asthma, eczema, and type 2 diabetes.” This sentence refers to three separate medical conclusions.

3. If several patients share the same condition, you could write: “The hospital recorded thirty new diagnoses of influenza in one week.” This means thirty instances of the same illness were identified in different people.

4. When comparing medical assessments, you might hear: “The second doctor’s diagnosis differed from the first, so the patient sought further tests before deciding between the two diagnoses.” This shows each conclusion is separate and can be evaluated on its own merit.

These examples show that the plural form is not simply a grammatical detail but a meaningful distinction in professional and everyday use.

What do academic studies say about this plural form?

Linguistic research confirms that irregular plural forms often survive in English when the words are used in technical or formal contexts.

Words from Greek and Latin tend to retain their original plural forms in scientific, medical, and legal language. This happens partly because professionals in those fields learn the traditional forms during their training, and partly because changing the forms could cause confusion in highly precise environments.

In a university linguistics course, you might learn that English has two main forces shaping plural forms: regularization, which tries to make all words fit the common rules, and preservation, which keeps older, less common patterns.

In everyday conversation, regularization wins more often, but in professional writing and education, preservation is preferred. That is why diagnoses remains the standard in medicine, science, and formal grammar.

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Understanding the pronunciation difference

Another area where people sometimes make mistakes is in pronunciation. The singular form diagnosis is pronounced “dahy-ag-NOH-sis.”

The plural diagnoses changes the ending sound and is pronounced “dahy-ag-NOH-seez.” The shift in sound comes from the Greek plural ending, and saying it correctly signals not only that you know the plural form but that you can also use it in a professional manner.

Why this rule is important to follow

Using the correct plural form matters because it communicates precision and credibility. In everyday language, incorrect plural forms might not cause much trouble, but in a medical report, research paper, or academic setting, an incorrect plural could suggest a lack of attention to detail.

For students, especially those studying medicine, nursing, or linguistics, using diagnoses correctly can help avoid misunderstandings and maintain professional standards.

Frequently Asked Questions

1. Is it ever correct to say “many diagnosis”?
No. This is considered incorrect in standard English. Even if you hear it in casual speech, it does not meet formal grammar rules. The correct form is always “diagnoses” when more than one is meant.

2. Do other languages use the same plural form?
Languages that borrow the word from English often keep “diagnoses” as the plural in medical contexts, although pronunciation may differ. In Greek, the plural is still “diagnoseis,” which is close to the English form.

3. Why keep the Greek form instead of regularizing it?
Keeping the Greek form maintains consistency across medical and academic language. It also avoids creating similar-looking words that could cause confusion. In addition, academic tradition values preserving original forms to show the link between English and classical languages.

4. Can “diagnoses” ever mean something else?
No. The word always refers to the plural of diagnosis, and its meaning stays the same regardless of the context. Only the subject of the diagnosis changes, such as diseases, technical problems, or errors in a system.

5. Does this rule apply to all words ending in “-is”?
Not all. While many Greek-derived “-is” words follow the pattern, some Latin-derived words may have different plural forms or follow the regular English “add s or es” pattern. The best way to know is to check a reputable dictionary or grammar guide.

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Final Thoughts

The plural of diagnosis is diagnoses, and knowing this form is important for clear and accurate communication, especially in medicine, research, and education.

This form comes directly from Greek grammar, and English retains it for precision and tradition. Using the correct plural not only shows respect for language history but also ensures that you are understood in professional and academic settings.

Once you understand the rule, you can confidently use it in both speech and writing, avoiding the common mistake of adding an extra “s” to make “diagnosises.” Remember that accuracy in language is just as important as accuracy in the information you are communicating.