BACKGROUND: Recent evidence suggests that a low carbohydrate (LC) diet may be equally or more effective for short-term weight loss than a traditional low fat (LF) diet; however, less is known about how they compare for weight maintenance. The purpose of this study was to compare body weight (BW) for participants in a clinical weight management program, consuming a LC or LF weight maintenance diet for 6 months following weight loss. METHODS: Fifty-five (29 low carbohydrate diet; 26 low fat diet) overweight/obese middle-aged adults completed a 9 month weight management program that included instruction for behavior, physical activity (PA), and nutrition. For 3 months all participants consumed an identical liquid diet (2177 kJ/day) followed by 1 month of re-feeding with solid foods either low in carbohydrate or low in fat. For the remaining 5 months, participants were prescribed a meal plan low in dietary carbohydrate (~20%) or fat (~30%). BW and carbohydrate or fat grams were collected at each group meeting. Energy and macronutrient intake were assessed at baseline, 3, 6, and 9 months. RESULTS: The LC group increased BW from 89.2 +/- 14.4 kg at 3 months to 89.3 +/- 16.1 kg at 9 months (P = 0.84). The LF group decreased BW from 86.3 +/- 12.0 kg at 3 months to 86.0 +/- 14.0 kg at 9 months (P = 0.96). BW was not different between groups during weight maintenance (P = 0.87). Fifty-five percent (16/29) and 50% (13/26) of participants for the LC and LF groups, respectively, continued to decrease their body weight during weight maintenance. CONCLUSION: Following a 3 month liquid diet, the LC and LF diet groups were equally effective for BW maintenance over 6 months; however, there was significant variation in weight change within each group.