In spite of numerous studies in the literature, it is still questionable as to whether the isocaloric exchange of carbohydrate and fat, in the form of a diet, leads to different degrees of weight loss. In comparative studies, obese patients given a low-carbohydrate (4.14 MJ [1000 kcal]) formula diet (diet Ia) lost 14.0 +/- 1.4 kg and those given an iso-energetic high-carbohydrate diet (diet Ib) 9.8 +/- 0.9 kg. The degree of weight loss was significantly different. Daily weight losses were 362 g and 298 g respectively. Comparative studies of high and low-carbohydrate (7.83 MJ [1900 kcal]) formula diets (diets IIa and b) with a greater number of calories did not show any significant difference. However, there was a greater mean weight loss with the low-carbohydrate diet (351 g/day) compared with that under the high-carbohydrate diet (296 g/day). Evaluation of 117 patients treated with formula diets resulted in a weight loss of over 9 kg in 102 obese patients and over 18 kg in 52 patients. The good response to the low-carbohydrate diet was partly responsible for the successful therapy.