We describe seven years' experience with the University of Cape Town lenticular mitral valve prosthesis in 122 patients. All the patients had severe mitral valve disease. In 98 severe mitral stenosis was present with or without incompetence and in 24 the dominant or sole lesion was mitral incompetence. Other valves, particularly the tricuspid, were also frequently affected. The disability was severe or total in almost every patient. One hundred and five patients were discharged from hospital, and in 90 per cent of these the clinical improvement was most gratifying, with the disappearance of pulmonary oedema, paroxysmal dyspnoea, angina pectoris, and congestive cardiac failure. Return to full normal activity including physical work was the rule. The hospital mortality was 14 per cent and a further 38 per cent died during the follow-up period. The major post-operative complication was systemic embolism which could occur at any time after operation. The most important factor influencing the frequency of this complication was the nature of the valve seat. A bare steel seat was associated with a 100 per cent embolism, and a significant reduction occurred when a cloth-covered seat of Dacron-velour was introduced. Anticoagulant therapy appeared to prevent large or fresh clots but had no effect on the deposition of fibrin or platelet thrombi. The only other factor of importance was the age of the patient: after the age of 50 life expectancy and trouble-free long-term survival was reduced.