Patterns of performance on 22 neuropsychological tests were examined in 58 patients with presumed dementia of the Alzheimer type (DAT) and 58 patients with presumed multi-infarct dementia (MID). Few differences between patterns of performance in DAT and MID were found when overall level of performance was taken into account. Within the DAT group, there were some age-related differences in performance that in themselves might support other studies suggesting clinical and neuropathological differences between subtypes of DAT that are related in part to the age at which the condition occurs. However, similar age-related patterns were found in the MID group and the DAT and MID groups did not differ significantly in this respect. Considerable inter-individual variability in pattern of performance was apparent within each group. A proportion of patients were re-assessed 10 months after initial assessment: the extent of decline over time was not predictable from patients' characteristics or test performances at initial assessment. 'De-differentiation' in patterns of impairment, over time or with increasing severity of impairment, was not observed.