INTRODUCTION: The purpose was to investigate the long-term effect of ad libitum low-fat, high-carbohydrate diets with mainly simple or complex carbohydrates on body weight and blood lipids. MATERIAL AND METHODS: A six-months controlled randomised multicentre trial with the participation of five European centres. A total of 398 moderately obese subjects (BMI: 30.4 +/- 2.7 kg/m2, mean +/- SD) were randomised to four groups: Control diet group with normal macronutrient intake, a low-fat/high simple carbohydrate group, a low-fat/high complex carbohydrate group and a seasonal control group. RESULTS: After six months we found a drop in body weight of 1.7 kg (p < 0.05) in the simple and of 2.6 kg (p < 0.001) in the complex carbohydrate group compared with the control diet group. Fat mass decreased by 1.9 kg (p < 0.05) and 2.4 kg (p < 0.001) in the simple and complex carbohydrate group, respectively, compared with the control diet group. There were no significant changes or group differences in fasting blood lipids, glucose, insulin, or leptin. DISCUSSION: The results show that it is favourable to replace dietary fat by carbohydrates (simple or complex) in relation to body weight regulation. No detrimental effects were seen on blood lipids in contrast to previous contentions. Our results underline the importance of a low-fat/high-carbohydrate diet in the management of obesity and the ensuing health problems.