BACKGROUND Little is known about the composition and source of weight loss after bariatric surgery for morbid obesity. OBJECTIVE This study was undertaken to determine changes in weight, body mass index (BMI), lean body weight (LBW), fat weight (FW) and left ventricular cardiac mass (LVM) following vertical banded gastroplasty (VBG). METHODS After VBG for morbid obesity, 26 women and four men (mean age = 39.1 years) were weighed and had body composition analysis undertaken at intervals. Thirteen patients underwent echocardiography preoperatively and 1 year postoperatively to determine change in LVM and LVM index. RESULTS Over 12 months there was significant weight loss for all weight parameters examined (p < 0.05). Fat weight loss was most significant; total weight loss and reduction of BMI were significant but less so than fat loss (Wilcoxon's signed ranks test). LBW loss had the smallest contribution to weight loss (p < 0.0001). There was a significant loss of LVM and posterior cardiac wall thickness (p < 0.05). CONCLUSIONS VBG can lead to loss of lean body weight and left ventricular mass, and more dramatically, fat weight, body weight, and BMI. Cardiac mass and lean body mass are preferentially conserved relative to body fat with weight loss after VBG.