The results of operation in all patients with univentricular heart and an obstructed anterior subaortic outlet chamber who were operated on utilizing extracorporeal circulation at the Mayo Clinic from 1973 through 1983 were reviewed. Ten of the 18 patients died during the immediate postoperative period and there was one late death. Factors significantly related to operative and immediate postoperative mortality were age at operation, cardiothoracic ratio on X-ray examination, degree of ST depression on electrocardiogram and pressure gradient across the outlet foramen at catheterization. Autopsy in eight cases revealed significant hypertrophy of ventricular myocardium and a small outlet foramen that was considered stenotic relative to either body surface area or aortic root area. The ventricular myocardium showed histologic changes of chronic ischemia that predated the surgical procedure.