What Is the Plural of Diagnosis? Meaning, Spelling, and Examples

The plural of diagnosis is diagnoses. The ending changes from -is to -es rather than simply adding an s.

• Singular: diagnosis
• Plural: diagnoses

For example, a doctor may make one diagnosis or consider several possible diagnoses. Forms such as diagnosises and diagnosies are incorrect.

Diagnosis vs. Diagnoses

Diagnosis refers to one identified illness, condition, or problem. Diagnoses refers to two or more.

Form	Meaning	Example
Diagnosis	One identified condition or problem	The doctor confirmed the diagnosis.
Diagnoses	Two or more identified conditions or problems	The specialists discussed several possible diagnoses.

The word is most often used in medicine, but it can also describe the identification of other problems.

• The mechanic’s diagnosis revealed a faulty battery.
• The technicians offered different diagnoses of the network problem.

Why Is the Plural Diagnoses?

Diagnosis belongs to a group of words derived from Greek that form their plurals by changing the ending -is to -es.

Singular	Plural
Analysis	Analyses
Basis	Bases
Crisis	Crises
Diagnosis	Diagnoses
Hypothesis	Hypotheses
Prognosis	Prognoses

This spelling pattern explains why diagnosises is not correct. The original ending changes instead of having another ending added to it.

How to Pronounce Diagnosis and Diagnoses

The singular and plural forms have different final sounds. Pronunciation can vary slightly by accent, but they are commonly pronounced as follows:

• Diagnosis: dye-ug-NOH-sis
• Diagnoses: dye-ug-NOH-seez

The final syllable of diagnosis sounds like “sis,” while the final syllable of diagnoses sounds like “seez.”

Examples of Diagnosis and Diagnoses in Sentences

Diagnosis in a Sentence

Use diagnosis when referring to one condition, conclusion, or identified problem.

• The doctor made a diagnosis after reviewing the test results.
• She received a diagnosis of asthma as a child.
• An accurate diagnosis can help guide treatment.
• The mechanic’s diagnosis identified a problem with the engine.
• The initial diagnosis was later confirmed by a specialist.

Diagnoses in a Sentence

Use diagnoses when referring to more than one.

• The two doctors reached different diagnoses.
• Similar symptoms can point to several possible diagnoses.
• The researchers compared the original diagnoses with later test results.
• The hospital reviewed the patients’ diagnoses before changing their treatment plans.
• The specialists considered several diagnoses before ordering more tests.

Diagnoses as a Noun and a Verb

Diagnoses can be a plural noun or a form of the verb diagnose. Its meaning depends on its role in the sentence.

Diagnoses as a Plural Noun

As a noun, diagnoses means more than one diagnosis.

• The doctors recorded their diagnoses in the patients’ files.
• Several diagnoses were considered before the results arrived.

Diagnoses as a Verb

As a verb, diagnoses describes the act of identifying a condition or problem. It is used with a singular subject such as he, she, or the doctor.

• The doctor diagnoses and treats respiratory conditions.
• She diagnoses equipment failures for the engineering company.

Use	Example
Plural noun	The doctors made several diagnoses.
Verb	The doctor diagnoses several conditions each week.

What Are the Possessive Forms?

The singular possessive is diagnosis’s, while the plural possessive is diagnoses’.

• Singular possessive: The diagnosis’s accuracy was questioned.
• Plural possessive: The diagnoses’ accuracy was reviewed.

Although these forms are grammatically correct, they can sound awkward. In many cases, rewriting the sentence is more natural:

• The accuracy of the diagnosis was questioned.
• The accuracy of the diagnoses was reviewed.

Final Answer

The plural of diagnosis is diagnoses. Change the ending from -is to -es.

• One diagnosis
• Two or more diagnoses

Remember that diagnoses can also be a verb, as in “The doctor diagnoses illnesses.” The sentence structure makes its meaning clear.