Objective: To seek differences in the pattern of decline between groups of patients with early-onset dementia of varying aetiologies (clinically defined) on the basis of the investigation of their different clinical profiles.Design: A cohort of 126 live patients with dementia diagnosed before the age of 65, of various aetiologies, were identified using the Lothian Psychiatric Case Register. Each person was seen for two assessments approximately 1 year apart.Setting: Patients were either in long-term care (NHS or other) or at home, usually in the care of relatives.Patients: Of 126 cases (53 male, 73 female, mean age at referral 58 years), 114 met the diagnostic criteria for DSM-III-R dementia: 60 Alzheimer's type dementia; 13 multi-infarct dementia; 14 alcohol-related dementia; with 25 in a mixed group of overlapping categories and two in 'other' dementia types. By the second assessment, 18 cases had died and two cases refused reassessment.Measures: The data collected included: demographic; behavioural and psychopathological; neurocognitive; neurological; and genetic. Change was assessed.Results: The alcohol group was distinguished from the other groups by its generally milder profile of impairment and a pattern of change indicating some areas of improvement in the follow-up period.Conclusions: There are limitations in the instruments used to measure change in this group of patients; at present, measures of change in dementing illnesses cater predominantly for an elderly population with Alzheimer's disease. [ABSTRACT FROM AUTHOR]