OBJECTIVE: To investigate the long-term effects of changes in dietary carbohydrate/fat ratio and simple vs complex carbohydrates. DESIGN: Randomized controlled multicentre trial (CARMEN), in which subjects were allocated for 6 months either to a seasonal control group (no intervention) or to one of three experimental groups: a control diet group (dietary intervention typical of the average national intake); a low-fat high simple carbohydrate group; or a low-fat high complex carbohydrate group. SUBJECTS: Three hundred and ninety eight moderately obese adults. MEASUREMENTS: The change in body weight was the primary outcome; changes in body composition and blood lipids were secondary outcomes. RESULTS: Body weight loss in the low-fat high simple carbohydrate and low-fat high complex carbohydrate groups was 0.9 kg (P <0.05) and 1.8 kg (P <0.001), while the control diet and seasonal control groups gained weight (0.8 and 0.1 kg, NS). Fat mass changed by - 1.3 kg (P <0.01), - 1.8 kg (P <0.001) and + 0.6 kg (NS) in the low-fat high simple carbohydrate. low-fat high complex carbohydrate and control diet groups, respectively. Changes in blood lipids did not differ significantly between the dietary treatment groups. CONCLUSION: Our findings suggest that reduction of fat intake results in a modest but significant reduction in body weight and body fatness. The concomitant increase in either simple or complex carbohydrates did not indicate significant differences in weight change. No adverse effects on blood lipids were observed, These findings underline the importance of this dietary change and its potential impact on the public health implications of obesity.