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What Is the Plural of Diagnosis?

Could you give the correct word for the plural of diagnosis? This question can sometimes trip writers, speakers, and even editors. In today’s review we’ll give you the answer, as well as touch on the reason these types of words can be tricky to form and remember. We’ll also share a few tips to help you keep them straight in the future.

The Plural of Diagnosis

The plural form of the word diagnosis is diagnoses. Here is an example of the word being properly used:

Dr. Martin examined several patients and found they had similar diagnoses.

Martha saw two doctors and received two different diagnoses, so she is going to consult a third physician.

Now we have the plural form of the word diagnosis, let’s further consider what makes pluralizing such words so challenging, even for those with a good grip on everyday grammar.

Pluralizing English Words Can Be Difficult

In many instances you can create a plural form of an English word just by adding an -s or -es ending. For example:

apple becomes apples

beach becomes beaches

tree becomes trees

At the same time, many current words in American English have roots in different languages, such as Latin, German, and Greek. Some words also don’t sound right when pluralized by normal methods, or they might be difficult to spell.


alumnus becomes alumni (not alumnuses)

story becomes stories (not storys)

mouse becomes mice (not mouses)

On top of that, some words, such as sheep, don’t get changed at all:

One sheep is walking

Two sheep are walking.

When you include these exceptions, you find that using certain words correctly calls for memorizing them. This can sometimes be a challenge for those who are learning American English or trying to enhance their grammar. A better approach than memorization can be to identify the similarities of word endings. For instance, many groups of words with similar endings are often pluralized in the same way.


tomato > tomatoes, potato > potatoes

datum > data, stratum > strata

With that understood, patterns will begin to emerge as you master the different types of words you frequently use. With just a little practice, you can learn to pluralize most if not all of them. You’ll also learn to spot the exceptions when they appear.

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