- A leaking anastomosis between the proximal stomach and the jejunum is a serious postoperative complication of the gastric bypass operation. Associated intra-abdominal sepsis carries a high mortality rate. Early diagnosis, particularly at the time of operation, and remedial treatment reduce mortality. We have devised a method for the detection of leaks, using nasogastric infusion of methylene blue with an i.p. sump suction catheter. In our initial group of 150 patients, where this method was not used, 5 patients had an anastomotic leak and 2 died of fulminant sepsis. The presence of a leak in these cases was diagnosed on clinical grounds and confirmed in all five by Gastrografin swallow between the 3rd and 5th postoperative day. In a later group of 250 patients, methylene blue was routinely used intra-operatively and twice daily thereafter to verify the integrity of the anastomosis. Of this group, seven patients developed leaks, five of which were diagnosed intra-operatively while the other two were diagnosed on the 1st postoperative day. There was no mortality in this group. The use of methylene blue provides early detection of a leaking anastomosis and thus may significantly reduce the morbidity and mortality of the gastric bypass operation.