To determine the effect of a 6-month very low carbohydrate diet program on body weight and other metabolic parameters.Fifty-one overweight or obese healthy volunteers who wanted to lose weight were placed on a very low carbohydrate diet (<25 g/d), with no limit on caloric intake. They also received nutritional supplementation and recommendations about exercise, and attended group meetings at a research clinic. The outcomes were body weight, body mass index, percentage of body fat (estimated by skinfold thickness), serum chemistry and lipid values, 24-hour urine measurements, and subjective adverse effects.Forty-one (80%) of the 51 subjects attended visits through 6 months. In these subjects, the mean (+/- SD) body weight decreased 10.3% +/- 5.9% (P <0.001) from baseline to 6 months (body weight reduction of 9.0 +/- 5.3 kg and body mass index reduction of 3.2 +/- 1.9 kg/m(2)). The mean percentage of body weight that was fat decreased 2.9% +/- 3.2% from baseline to 6 months (P <0.001). The mean serum bicarbonate level decreased 2 +/- 2.4 mmol/L (P <0.001) and blood urea nitrogen level increased 2 +/- 4 mg/dL (P <0.001). Serum total cholesterol level decreased 11 +/- 26 mg/dL (P = 0.006), low-density lipoprotein cholesterol level decreased 10 +/- 25 mg/dL (P = 0.01), triglyceride level decreased 56 +/- 45 mg/dL (P <0.001), high-density lipoprotein (HDL) cholesterol level increased 10 +/- 8 mg/dL (P <0.001), and the cholesterol/HDL cholesterol ratio decreased 0.9 +/- 0.6 units (P <0.001). There were no serious adverse effects, but the possibility of adverse effects in the 10 subjects who did not adhere to the program cannot be eliminated.A very low carbohydrate diet program led to sustained weight loss during a 6-month period. Further controlled research is warranted.