The purpose of this study was to investigate the occurrence of multiuse of prescription drugs and its major determinants among Finnish adults. The data were based on three nationally representative and intercomparable health surveys in 1976, 1978-80, and 1987. The study populations were 16,413 in 1976, 13,138 in 1987, and 7,217 in 1978-80. The simultaneous use of at least five prescription drugs was the main outcome measure. In 1976, 4.7% of the study population, and 6.3% in 1987 were multiusers. Of all users of prescription drugs, the proportion of multiusers was 14% both in 1976 and 1987. In 1987, the elderly comprised 55% of all multiusers, whereas their share in 1976 was 44%. The strongest predictor of the multiuse was, as expected, chronic morbidity. Of the various diagnostic groups, the main determinants were cardiac insufficiency, hypertension, asthma, mental disorders, and coronary heart disease. Even allowing for chronic morbidity, age was significantly associated with multiuse, but sex was not. The number of visits to a primary care physician correlated also independently with the multiuse. These findings indicate that the elderly may in part get prescriptions indiscriminately.