In most prospective studies involving older subjects, weight loss is associated with increased mortality. The authors examined the characteristics and health status of middle-aged men from 24 towns (the British Regional Heart Study), who reported that they had lost weight intentionally or unintentionally as they aged. Questionnaires were completed at screening (1978-1980), 5 years later, and in 1992 and 1996. This paper concerns the 4,713 men who reported their weight in 1992 and 1996 and their perceived weight change over this period. Weight loss was reported by 847 men (18%), of whom 39% stated that it was intentional. Compared with intentional weight loss, unintentional weight loss was associated with lower social class, more smoking, less obesity, and less physical activity and with the highest rates of "poor health,"long-standing disability, cancer, and respiratory diseases. The rates of recalled heart attack, stroke, diabetes, and cardiovascular hospitalization were higher than among men whose weight was stable. Men who lost weight intentionally had been more obese than those who lost weight unintentionally and had high rates of obesity-related conditions. Thus, both intentional and unintentional weight loss may follow the development of disease. These findings probably account for the lack of benefit and the increased mortality associated with weight loss observed in most large-scale prospective studies.