Thirty three patients having an anastomosis to the rectum were studied prospectively to determine the usefulness of intra-operative air testing. The pelvis was filled with saline and the anastomosis was distended with air. Any leaks demonstrated were oversewn. A Gastrografin enema was performed on all the patients on the eighth postoperative day. Overall, there were 4 (12%) clinical leaks and 10 (30%) radiological leaks. In six patients with air leaks demonstrated at operation, two had a radiological leak, one of which presented clinically as well. We believe that in the other four patients a potential leak was probably avoided as a result of intra-operative air testing. This technique is simple, safe and effective and probably helps reduce leakage after colorectal anastomosis.