Eighty-two patients underwent colorectal anastomosis using the circular mechanical stapling instrument. Integrity of the anastomosis was assessed intraoperatively by the air test and examination of doughnuts for completeness. A proctogram using a water soluble contrast preparation was performed in 79 patients postoperatively. In 68 patients doughnuts were complete and no air leak was found in any case. Postoperatively three clinical and one radiological leak occurred. In 14 patients at least 1 doughnut was incomplete. Of these, 4 had an air leak whilst 10 did not. There was no leak postoperatively in these 10 patients but 2 of the 4 patients with an intraoperative air leak developed a radiological leak postoperatively. The data suggest that the air test is useful since absence of air leakage in cases with an incomplete doughnut was followed by sound anastomotic healing. Further, the demonstration of air leakage enables immediate repair of the anastomosis perhaps avoiding subsequent clinical leakage.