BACKGROUND: a prospective randomized study was undertaken to compare the outcome of vertical banded gastroplasty (VBG) and gastric bypass (GBP) in patients with clinically severe obesity. METHODS: eligibility criteria included Class IV obesity, < 50 years old and a history of at least one attempt of non-operative weight loss. Patients were managed conservatively for 3 months prior to surgery. Patients were followed post-operatively and monitored for early and late complications and their weight loss outcome for up to 5 years. RESULTS: 44 patients were recruited. Two patients withdrew within 4 weeks and were excluded. Twenty subjects had a GBP and 22 a VBG. There were no significant differences with respect to age, gender, maximum or pre-operative weight between the groups (p > 0.05). Patients who underwent GBP demonstrated significantly greater post-operative weight loss (p < 0.05) which was apparent from 6 months onwards. There were no deaths, pulmonary emboli, post-operative leaks or wound dehiscence. There were no instances of staple-line disruption. Symptomatic ulcer disease, confirmed endoscopically, developed in 25% of GBP patients. Nine patients developed gallstones post-operatively of whom five were in the VBG and four in the GBP group. CONCLUSIONS: weight loss following GBP was maintained, while VBG patients slowly regained.