OBJECTIVE: The psychopathology associated with early-onset dementia of the Alzheimer type was investigated by comparing the prevalence of psychiatric symptoms in younger subjects (mean age = 59 years) who had very mild or mild dementia with that in older adults (mean age = 72) whose dementia was of equivalent severity. METHOD: Nondemented comparison subjects and persons with very mild or mild dementia of the Alzheimer type were recruited to participate in longitudinal studies. All subjects met strict inclusion and exclusion criteria. Information pertaining to personality changes, affective symptoms, and psychotic symptoms was included in the 90-minute semistructured, physician- administered interview, which was used to assign a clinical dementia rating according to published guidelines. The younger group were age 64 or younger and consisted of 20 nondemented subjects, 11 subjects with very mild dementia, and 18 subjects with mild dementia. The older group, described previously, were 64-83 years old and consisted of 83 nondemented subjects, 41 persons with very mild dementia, and 68 subjects with mild dementia. RESULTS: The psychopathology in the younger subjects was similar to that in the older group. Personality changes occurred in over 80% of the younger persons with very mild illness. Psychotic symptoms were present in over 40% of the younger persons with mild illness but were rare in the group with very mild dementia. CONCLUSIONS: Similar patterns of psychopathology in younger and more elderly persons with dementia of the Alzheimer type support the suggestion that these changes are direct effects of the illness on the CNS. Increased attention to documenting these noncognitive symptoms and studying various treatments is urgently needed.