Sixteen smokers (eight men and eight women) were studied before and six weeks after attending a series of anti-smoking clinics. Mean weight gain for the ten subjects who gave up smoking was 1.36 kg (P less than 0.005) and there was no significant change in body weight of subjects who did not give up smoking. There was a 4 per cent drop in resting metabolic rate of the smokers who successfully gave up (n = 9), but this was only significant when the data were expressed per kg body weight (P less than 0.05). Mean energy intake increased by 6.5 per cent but this change was not significant. Smoking a single cigarette significantly increased the metabolic rate for 30 min by 3 per cent compared with a control condition of sham smoking (n = 15). Thus the discrepancy in body weight between smokers and non-smokers appears to be due to a combination of reduced food intake and the thermogenic effects of smoking.