The prevalence of dementia disorders were determined among 3,523 institutionalized patients aged 65 years or more in the county of Västerbotten. Approximately one-third of the clientele were considered demented. The prevalence was calculated to be 3.7% of the population in the county. The frequency of dementia disorders among the patients in the somatic long-stay wards, the nursing homes, and the mental hospital was rather similar (55-65%). A surprisingly high percentage of demented (17.1%) was found in the homes for the aged. The work load was highest in the somatic long-stay wards and in the nursing homes. Patients in the mental hospital, however, differed markedly from those in other institutions with higher scores for aggressive and disturbing behaviour, which accounted for the large number of patients in the mental hospital with maximal work load. The findings are discussed with regard to future institutional care of demented patients and point out the need for better education in psychiatric care for the staff.