Abstract A four-year clinical evaluation of 133 patients with Lillehei-Kaster pivoting-disc valves has shown that patients with an aortic valve prosthesis have remained free from valve-related problems. Patients with a mitral valve implanted have a satisfactory survival record as calculated by actuarial methods; however, a 10% incidence of valve thrombosis was experienced. Analysis suggests that this may be related to inadequate anticoagulation or use of an inappropriate suturing technique or both. The patients enjoyed significant clinical improvement following operation, with no evidence of hemolysis. A history suggestive of postoperative embolization was present in only 1 patient in this series.