Left-sided colorectal anastomoses sometimes leak despite the best efforts of the surgeon. The cause is thought to be multifactorial but imperfections in the technique of performing the anastomosis must play some part. A method for testing the integrity of left-sided colorectal anastomoses has been developed using intraluminal distension. Twenty-one patients were studied and defects in the anastomosis were found in five (24%). These defects were rectified at the time of operation. We conclude that this method is useful for demonstrating imperfections in anastomotic technique and we suggest a distending pressure of 25 cm saline. We suggest that the method may be particularly useful for the surgeon in training.