The incidence of postoperative complete heart block in a series of 124 cases of replacement of the aortic valve by a ball valve prosthesis was 12.9% (16 of 124). In all but one case, complete heart block was treated by pacing the heart with an artificial pacemaker. The complete heart block was temporary in seven cases and persistent in nine. Six of the patients with acute persistent complete heart block died within ten days of operation, and one patient died on the forty-ninth postoperative day. Two patients are alive, both with implanted pacemakers. Five of the seven patients who resumed their preoperative rhythm survived. Among the 16 patients with some form of complete heart block postoperatively, nine died (56.2%). The basis for surgical induction of complete heart block when placing a ball valve prosthesis in the aortic orifice lies in the proximity of the bundle of His and its left branch to the aortic valve. In particular, the inferior aspect of the noncoronary cusp is closely related to major conduction tissue. Under the conditions of this operation, the conduction tissue may be strangulated or injured by sutures, or it may be injured when calcific material of the diseased aortic valve is removed. It is of interest that the incidence of postoperative complete heart block in this series was approximately the same with the Starr-Edwards sutured prosthesis (that is, 13%; 11 of 84 cases) as it was with the Magovern sutureless prosthesis (12.5%; 5 of 40 cases).