The aim of this study was to find out how individually prescribed moderate physical training influences aerobic power and body weight in obese middle-aged men and women. 210 persons with poor physical fitness and body mass index (BMI) over 25 kg/m2 were accepted. Of these 169 persons were without regular medication and in this group 95 persons trained actively for the complete follow-up period of 17 months while 74 persons dropped out during the study. Maximal oxygen consumption was estimated indirectly before training and after 2, 5, 11 and 17 months of training. The exercise physiologist prescribed an individual training program, which was checked every six weeks. The most popular sports were walking and skiing. Initial aerobic power was poor in both sexes (3.1 +/- 0.1 l/min in men and 2.3 +/- 0.1 l/min in women, mean +/- SE). In the actively training group it increased significantly after two months and remained on this improved level for the rest of the observation period (total increase 14% in men and 11% in women). Initial BMI was 29.8 +/- 0.5 kg/m2 in men and 29.1 +/- 0.4 kg/m2 in women. The greatest change in body weight in the actively training group also took place during the first months. The mean decrease of body weight was 4.8 +/- 0.8 kg in men and 3.3 +/- 0.6 kg in women after 17 months. In conclusion, obese middle-aged men and women initially in poor physical condition can benefit from an individually prescribed training program resulting in significantly increased aerobic power and a minor loss of overweight.