The effect of a fat-modified diet with 1100 kcal (4600 kJ) on the reduction of body weight and body fat was studied. A low-carbohydrate, high-fat diet with predominant animal fat (diet T) was compared with a low-carbohydrate, high-fat diet with predominant vegetable fat (diet P). Diet T was composed of 52.1% of the energy as fat, 20.7% as protein and 27.2% as carbohydrates. Diet P was composed of 54.1% of the energy as fat, 18.9% as protein and 27.0% as carbohydrates. More than two-thirds of the fat in diet T was butter, in diet P margarine. The study was carried out with 30 subjects (8 men) over two 21-day periods. With diet T, men had a mean weight loss of 7.1 kg = 338 g/d, of which 3.2 kg = 152 g/d were proved to be body fat, while women had a mean weight loss of 4.4 kg = 210 g/d, of which 2.3 kg = 110 g/d were proved to be body fat. With diet P, men had a mean weight loss of 7,6 kg = 362 g/d, of which were 3.9 kg = 186 g/d body fat, while women lost 3.8 kg = 181 g/d of body weight on average, of which were 2.0 kg = 95 g/d body fat. An improvement of blood pressure was also found. Significant differences of the reduction of body weight and body fat between the diet with mostly animal fat and the diet with mostly vegetable fat were not found.