Why is the plural of “diagnosis” not “diagnosises”?

The plural of diagnosis is diagnoses, not diagnosises. Diagnoses is considered an irregular plural, which doesn’t follow the pluralization rules of regular plural nouns.

Diagnosis is a Greek-derived word, meaning it follows the Greek rules of pluralization, so the “-is” is changed to “-es.”

Scribbr’s free Grammar Checker can help you make sure you’re using the correct plural forms of words.