Overweight and obesity have been examined in 7735 middle-aged men in 24 British towns. Half the men exceeded the body mass index (BMI) range associated with minimum mortality (20-25 kg/m2). Social class differences in BMI were marked and obesity was more marked in manual workers. The association of reduced BMI with cigarette smoking and of increased BMI with stopping smoking was most clearly seen in manual workers. With increasing alcohol intake, BMI increased progressively, but the effect in the heaviest drinkers was probably diminished by concurrent heavy smoking. Mean BMI decreased with increasing levels of physical activity. There was considerable variation in the rate of obesity between the towns, from 11 to 28 per cent, determined to some extent by social class. Positive associations were observed between BMI and the presence of ischaemic heart disease, high blood pressure, gout, arthritis and gallbladder disease but not with diabetes mellitus. Peptic ulcer was inversely related to BMI and bronchitis showed a curvilinear relationship. For these men, overweight or obesity is virtually 'normal', and a considerable health education effort will be needed to produce a leaner, healthier society.