A 4-6 mo study was conducted to examine effects of a very-low-calorie, high-protein diet and realimentation on energy expenditure, resting metabolic rate (RMR), and serum thyroid hormones of obese women aged 30-54 yr. Fifteen healthy women, greater than or equal to 126% ideal body weight, were placed on the diet (420 kcal/day) and lost an average of 1.1 kg/wk until a predetermined goal weight was attained. RMR, triiodothyronine (T3), and reverse T3 decreased significantly (p less than 0.05). Thyroxine remained unchanged. Upon gradual realimentation onto solid foods, all metabolic parameters increased significantly within 5 wk toward pre-diet baseline values, but RMR (kcal/h) and T3 values remained significantly below pre-study values. Estimates of mean energy expenditure, utilizing a technique based on energy intake and body composition changes, averaged 1719 kcal/day during the diet period.