BACKGROUND: The dumping syndrome that follows Roux-en-Y gastric bypass for morbid obesity is considered to, be the primary mechanism of improved weight loss as compared with the purely restrictive vertical banded gastroplasty. To evaluate the influence of dumping on post-operative weight loss, severity of dumping was determined using Sigstad's clinical diagnostic index. METHODS: One hundred and thirty seven gastric bypass and 19 gastroplasty patients were assessed 18-24 months following surgery. Sigstad's criteria for the dumping syndrome were met by 75.9% of gastric bypass and no gastroplasty patients. Among gastric bypass patients, no relationships were found between severity of dumping and weight loss, as measured by per cent of excess body weight loss or change in body mass index. Weight loss was significantly greater with gastric bypass than gastroplasty patients (72.5 compared to 47.9% of excess body weight loss). All gastroplasty and 24.1% of gastric bypass patients were classified as non-dumpers. The difference in weight loss between surgical procedures was not related to dumping: gastric bypass non-dumpers lost significantly more weight (69.1% excess body weight loss) than gastroplasty patients. CONCLUSIONS: This study fails to demonstrate a significant relationship between dumping severity and weight loss. It is inferred that the superior weight loss of gastric bypass compared to gastroplasty has some other etiology.