Mentally retarded obese in-patients were fed by low-energy diet (4.2-4.6 MJ) for 9 months. During this period, an average of 13 +/- 4.5 kg loss of body mass occurred in men and 16 +/- 2.7 kg in women. Anthropometric measurements were performed before starting the dietotherapy and in the ninth month. Changes of body fat could be followed well when calculated according to BMI. Less reliable results were obtained with skinfold thickness measurements, presumably due to body deformities. Results of clinical laboratory tests, which were carried out before starting the dietotherapy and in the fourth and seventh months, suggested that a low-energy-containing diet with balanced nutrient content and adequate protein intake did not impair protein metabolism, favourably affected serum triglyceride and cholesterol levels, but resulted in an unfavourable decrease in the HDL-cholesterol content.