Eight obese children and adolescents, mean age (+/- SD) 12.0 +/- 2.5 years, were treated for 3 weeks with a liquid formula very low calorie diet (VLCD), containing 320 kcal/1339 kj (44 g protein, 33 g carbohydrate, 0.9 g fat). Weight loss after 3 weeks was 8.0 +/- 1.8 kg resulting in 15.3 +/- 4.6% reduction of body overweight. During the dietary period no patient complained of hunger and no serious side effects were observed. Four patients achieved positive N-balance during the second week, all but one in the third week. Mean cumulative N-balance after 3 weeks was calculated to be -23.2 +/- 31.6 gN. Great interindividual variances were observed in the rate of N-loss during the course of the study. No significant correlation was found between cumulative N-balance and weight loss or initial body weight. Blood parameters remained unaffected, except for glucose and urea, which decreased slightly from 74.6 +/- 13.6 to 50.4 +/- 20.1 mg/dl and from 14.1 +/- 4.3 to 8.6 +/- 7.4 mg/dl, respectively. Uric acid concentrations increased slightly, three of eight patients had levels higher than 8 mg/dl and therefore were treated with allopurinol. Total serum protein decreased; serum albumin values did not change. The type of VLCD used in this study proved therapeutically useful in achieving rapid weight loss. Compared with VLCD containing 30% less protein and carbohydrate, a marked improvement of N-balance in 3 weeks could be achieved with the VLCD containing 1 g protein/kg IBW/day. This amount of protein seems to be necessary to obtain the nitrogen sparing effect in children and adolescents undergoing weight reduction with VLCDs.