The course of decline was studied in 16 patients with probable or definite dementia of the Alzheimer type (DAT) over 2.7 to 6.8 years from first to last evaluation. Overall severity of dementia was measured with the Wechsler Adult Intelligence Scale (WAIS), the Dementia Rating Scale (DRS), and the Mini-Mental State Examination (MMSE), at approximately annual intervals. An initial plateau phase, during which language and cognitive functions did not change for periods of 9 to 35 months, was observed in 5 patients who initially had an isolated memory impairment without significant impairment of nonmemory language or visuospatial function. Once nonmemory functions began to decline, the rate of decline was remarkably steady in most individual patients but varied markedly among patients. The initial rate of decline after the plateau phase, as measured with the WAIS and DRS, was a significant predictor of subsequent rate in individual patients (r = .66, p less than .01, and r = .67, p less than .01, for the WAIS and DRS, respectively). The MMSE was a less reliable measure of longitudinal change in dementia severity and did not predict future rates of decline (r = .29). These results demonstrate a biphasic trajectory of decline in patients with DAT. Stable interindividual differences in rate of decline may provide a basis for designing more sensitive studies of treatments intended to slow or halt the progress of DAT.