Impairment in naming visually presented objects was investigated in patients with a clinical diagnosis of Alzheimer's disease. Impaired object naming correlated with difficulty listing the names of objects from a specified semantic category and with erroneous selection of words semantically related to the correct names for objects in a name recognition test. These results suggest that patients with Alzheimer's disease have a semantic impairment characterized by inability to distinguish among objects that are members of the same semantic category, and that this impairment is associated with difficulty producing the names for objects. Semantic impairment was present in patients with normal ability to discriminate visually presented shapes, indicating that the semantic deficit in Alzheimer's disease occurs independently of abnormalities of visuospatial function. Patients tended to make errors on the same items in both confrontation naming and name recognition tests, suggesting that the semantic impairment in Alzheimer's disease involves loss of information about specific objects and their names.