Although the cognitive and psychiatric symptoms associated with Alzheimer disease have received increasing attention over the past decade, the study of insight in this illness has been relatively neglected. This paper reports on the relationship between level of insight and severity of dementia in a large sample of patients with Alzheimer disease, largely with mild to moderately severe dementia. The study is based on data from 670 patients with a research diagnosis of probable Alzheimer disease who were enrolled in the Consortium to Establish a Registry for Alzheimer's Disease (CERAD). The degree of insight was rated by the examiner on the basis of the patient's answers to questions probing awareness of his or her memory deficits or other symptoms of cognitive impairment. Severity of dementia was assessed using the Clinical Dementia Rating Scale, Short Blessed Test, Blessed Dementia Rating Scale, and Folstein Mini-Mental State Examination. Two-year longitudinal follow-up data on insight level and dementia severity were available for 148 of the 670 patients. Decreased level of insight correlated significantly with severity of dementia as measured by all rating instruments. For the patients followed for 2 years, 33.1% declined in level of insight from the entry level. This decline was statistically associated with more severe dementia as measured by the Blessed Dementia Rating Scale. This study confirms the generally accepted belief that patients with Alzheimer disease experience a progressive loss of insight as the severity of dementia increases. Due to methodologic limitations, we are unable to draw conclusions about loss of insight in patients with very mild cognitive impairment.