The effects of repeated periods of weight loss and regain on metabolic rate and body composition were investigated in 11 obese women (mean weight 81.98 kg, height 1.61 m, body mass index 31.44 kg/m2) studied for 18 weeks through three consecutive cycles of 2 weeks dieting followed by 4 weeks ad libitum eating. Weight loss was achieved by a very low energy diet (1861 kJ/day). Basal metabolic rate (BMR) was measured by whole-body indirect calorimetry and body composition by a variety of standard in vivo methods. During the three diet periods mean weight losses were 4.44, 3.29 and 2.98 kg although the mean overall weight loss from week 0 to 18 was only 5.93 kg. The proportion of weight lost as fat was estimated as between 67 and 105 per cent of the weight lost depending on the body composition methodology employed. Absolute BMR decreased in response to dieting by 545, 285 and 286 kJ/day. When corrected for body weight and FFM only the decreases in the first diet period were significant (P less than 0.05 and P less than 0.001 respectively). BMR had returned to normal following each 4-week ad libitum period and by the end of the study absolute BMR and BMR/kg FFM had not changed significantly, despite a significant loss of weight. Consequently BMR/kg was increased (P less than 0.01), indicative of the loss of adipose tissue. We have found no evidence in this group of obese women that weight cycling leads to a progressive decrease in BMR or increase in the proportion of body fat.