In two groups of obese patients with type 2 diabetes the effects of 2 different diet compositions were tested with regard to glycaemic control and bodyweight. A group of 16 obese patients with type 2 diabetes was advised on a low-carbohydrate diet, 1800 kcal for men and 1600 kcal for women, distributed as 20% carbohydrates, 30% protein and 50% fat. Fifteen obese diabetes patients on a high-carbohydrate diet were control group. Their diet, 1600-1800 kcal for men and 1400-1600 kcal for women, consisted of approximately 60% carbohydrates, 15% protein and 25% fat. Positive effects on the glucose levels were seen very soon. After 6 months a marked reduction in bodyweight of patients in the low-carbohydrate diet group was observed, and this remained one year later. After 6 months the mean changes in the low-carbohydrate group and the control group respectively were (+/-SD): fasting blood glucose (f-BG): -3.4 +/- 2.9 and -0.6 +/- 2.9 mmol/l; HBA1c: -1.4 +/- 1.1% and -0.6 +/- 1.4%; Body Weight: -11.4 +/- 4 kg and -1.8 +/- 3.8 kg; BMI: -4.1 +/- 1.3 kg/m_ and -0.7 +/- 1.3 kg/m_. Large changes in blood glucose levels were seen immediately. A low-carbohydrate diet is an effective tool in the treatment of obese patients with type 2 diabetes.