The Model 6120 ball valve prosthesis introduced in 1965 is still strongly supported as a mitral valve substitute in many centers around the world. A current reassessment of the performance of this prosthesis is therefore pertinent to current medical practice. In this institution since 1974, 227 Starr-Edwards caged ball valves have been implanted in the mitral position during isolated valve replacement. Two models of caged ball valves were used concurrently: the silastic ball valve in 108 patients (48%) and the composite strut "tract" valve in 119 (52%). Hospital mortality was 7%, and 8-year survival (standard error) was 74 (6%), with 100% follow-up, documenting 752 total patient-years. No late deaths were known to be valve related, and there were no cases of prosthetic thrombosis. The actuarial estimate of patients free from thromboembolism at 8 years was 89 (4%) with a linearized rate of 1.3% per year. At the most recent follow-up, 95% of the patients were in the New York Heart Association (NYHA) Classes I or II. These good results were partly due to an awareness at operation of ventricular outflow tract size requirements and to strict control of postoperative anticoagulation. We conclude that the Starr-Edwards ball valve is the mitral valve of choice in the young patient who is able to take anticoagulation drugs and has a left ventricular outflow tract of satisfactory size.