The reliability of the clinical diagnosis of dementia was estimated by comparing the diagnosis made at 1-year intervals on 55 consecutive subjects with suspected cognitive impairment seen at three different centers by neurologists and gerontologists. The diagnosis was based on history and clinical examination, the criteria of the Diagnostic and Statistical Manual of Mental Disorders (revised ed 3), the Modified Ischemic Score, and a computed tomographic scan. Fifty-two of 55 subjects were given the same diagnosis a year later indicating a reliability of 95%. The study shows that a diagnosis of dementia established by simple clinical criteria comparable to the NINCDS/ADRDA criteria affords sufficient reliability to allow the comparability of groups at different centers for purposes of research, including research on the evaluation of the efficacy of pharmacologic treatment.