The provisional diagnostic criteria for the Dementia Questionnaire for Persons with Mental Retardation (DMR), developed during a prior study, were evaluated in a 5-year longitudinal follow-up of 33 elderly institutionalized persons, aged 70 years and over, and 45 institutionalized persons with Down's syndrome, aged 35 years and over, with no dementia in the diagnosis at initial evaluation. During the study period, dementia was diagnosed according to DSM-III-R criteria in five elderly subjects and five subjects with Down's syndrome, whereas a diagnosis of possible dementia was made in two elderly subjects and three subjects with Down's syndrome. A DMR diagnosis based on the criterion increase over time of the sum of cognitive scores (SCS) > or = 7 points and/or of the sum of social scores (SOS) > or = 5 points' resulted in a sensitivity of 100% for both groups and a specificity of 73% in the elderly sub-group and 75% in the sub-group with Down's syndrome, independent of the (premorbid) intellectual level. A diagnosis based on a single completion of the DMR, using available information on former performance levels, also produced favourable results in the present study. However, this diagnostic approach is certainly not recommended for studies of larger samples because of the use of different methods measuring functional levels and different standards for levels of intellectual disability.