Body weight, body mass index (BMI), and percent excess weight loss are used to assess patient outcomes after bariatric surgery; however, they provide little insight into the true nature of the patient's weight loss. Body composition measurements monitor fat versus lean mass losses to permit interventions to reduce or avoid lean body mass loss after bariatric surgery. A retrospective review of patients who underwent bariatric surgery between 2002 and 2008 was performed. Patients underwent body composition testing via air displacement plethysmography before and after surgery (6 and 12 months). Body composition changes were assessed and compared with the BMI. Results include 330 patients (54 male, 276 female). Average preoperative weight was 139 kg, BMI was 50 kg/m(2), fat percentage was 55 per cent, and lean mass percent was 45 per cent. Twelve months after surgery average weight was 90 kg, mean BMI was 32 kg/m(2), fat percentage was 38 per cent, and lean mass percent was 62 per cent. Body composition measurements help monitor fat losses versus lean mass gains after bariatric surgery. This may give a better assessment of the patient's health and metabolic state than either BMI or excess weight loss and permits intervention if weight loss results in lean mass losses.