Four hundred twelve patients underwent gastric bypass for treatment of morbid obesity between 1981 and 1985 at the University of Florida Affiliated Hospitals. Thirty-four patients (8.2%) developed marginal ulcers, considerably higher than the 0-3 per cent ulcer occurrence commonly reported in the literature. Factors predisposing to ulcer formation include: (1) a large gastric pouch; (2) a vertically oriented pouch; and (3) staple-line dehiscence. Twenty-two of 34 patients (65%) with symptomatic marginal ulcers were noted to have staple-line disruption. Twenty-one of these patients (95%) eventually required operative therapy for their ulcers compared with four of 12 patients (33%) with an intact gastric staple line. Surgical therapy consisted of takedown of the Roux-en-Y limb with resection of the ulcer and gastrogastrostomy. Staple-line dehiscence is a significant etiologic factor in the development of marginal ulcer following gastric bypass and when present constitutes an indication for reoperation.