To investigate the nature and rate of cognitive decline in dementia of the Alzheimer's type (DAT), 44 patients with probable or possible DAT (mean age 73.9, range 59-87 years) were given a comprehensive neuropsychological assessment. Twenty-five patients were retested 6 months later, and 11 were tested for the third time another 6 months later. Some tests of mental flexibility and double conceptual tracking ability were beyond the capabilities of the patients, and were left out of the statistical analysis because of floor effect. Poor initial performance on tests of attention, concentration, response speed, general cognitive ability, verbal memory, constructional skills, and perceptual closure ability resulted in nonsignificant changes over time (paired t tests). Tests of expressive speech, visuoperceptual functions and nonverbal memory, on the other hand, showed a significant deterioration over time. These findings indicate that when patients with DAT first receive medical attention, many neuropsychological functions are so severe impaired that there is little room for further decline. To describe the longitudinal course of cognitive decline in DAT, tests should be limited to those that assess the more resistant cerebral functions, and are more sensitive to progressive changes.