After a successful trial of bilio-pancreatic bypass in dogs, a clinical study has been completed in 18 patients followed for more than 1 year. The operation has been modified to achieve the best weight reduction, and forming the bilio-pancreatic tract of equal length to the alimentary tract with a short common ileal tract, the average weight loss as a percentage of the preoperative body weight was 24.1 +/- 5.4 per cent (mean +/- s.d.) at 6 months and 33.7 +/- 4.1 per cent at 12 months. The only immediate complication was a wound dehiscence, and there were no late complications. Liver function studies showed the absence of hepatic deterioration and liver biopsies showed improvement of liver morphology 1 year after the operation. It is suggested that this procedure may be an alternative to jejuno-ileal bypass in the management of obesity.