Thromboembolism, haemolysis, prosthetic endocarditis and paravalvular leakage are the commonest complications of introduction of mechanical heart valve prosthesis. Malfunction of the cusps inside the sewing ring is a rare but serious condition, which is due to partial or total prevention of the free movements of the cusps or valves resulting in symptoms of valvular stenosis or incompetence. Twenty-six valvular prosthesis (2.3%) were replaced on account of malfunction. Fifteen out of the 26 patients were submitted to emergency operation and 11 to elective intervention. Four out of the 26 patients died preoperatively or during the postoperative course, corresponding to a lethality of 15%. The operative lethality was associated with emergency operations exclusively. The authors conclude that patients with previously well functioning valvular prosthesis who develop symptoms of cardiac failure should be referred to the cardiological centre where the operation was carried out as rapidly as possible instead of institution of traditional medical treatment.