The natural history of obesity was examined in a nation-wide sample of 1,084 obese men and 1,367 obese women aged 37-59 years who were recruited into a registry of potential subjects for an intervention trial. The registry and intervention studies are jointly referred to as SOS ('Swedish Obese Subjects'). In the registry, the mean body mass index was 37.7 kg/m2 in men and 40.9 kg/m2 in women. Descriptive information on subjects' weight histories and the relative weights of their biological parents was collected by means of a self-administered questionnaire. At a physical examination shortly thereafter, weights, heights and selected cardiovascular risk factors were measured in the fasting state. Virtually all subjects reported weight loss attempts in the past, men and women reported having gained weight during adulthood at a considerably higher rate than that observed in population-based samples. Significant correlations were observed between relative weights of obese males and both of their biological parents, but not between obese women and either of their parents. Indices of medical risk were then examined in relation to individual weight histories. Familial predisposition did not relate to most aspects of current medical risk. However, later-onset obesity tended to be associated with greater cardiovascular risk, while prior weight loss was predictive of an improved risk factor profile. These latter associations were not dependent on a subject's current degree of obesity and were particularly consistent with respect to fasting insulin level.