Agrammatism is a characteristic of non-fluent aphasia. Individuals with agrammatism present with speech that is characterized by containing mainly content words, with a lack of function words. For example, when asked to describe a picture of children playing in the park, the affected individual responds with, "trees..children..run." People with agrammatism may have telegraphic speech, a unique speech pattern with simplified formation of sentences (in which many or all function words are omitted), akin to that found in telegraph messages. Deficits in agrammaticism are often language-specific, however—in other words, "agrammaticism" in speakers of one language may present differently from in speakers of another.

Errors made in agrammatism depend on the severity of aphasia. In severe forms language production is severely telegraphic and in more mild to moderate cases necessary elements for sentence construction are missing. Common errors include errors in tense, number, and gender. Patients also find it very hard to produce sentences involving "movement" of elements, such as passive sentences, wh-questions or complex sentences.

Agrammatism is seen in many brain disease syndromes, including expressive aphasia and traumatic brain injury.