Valve thromboses were observed in nine patients, seven females and two males, aged 46 years on average, between 1/2 and 5 1/2 years after replacement of a mitral valve (n = 6) or an aortic valve (n = 3) by a tilting disc valve (Björk-Shiley, n = 6; Lillehei-Kaster n = 3). In eight cases reoperation was necessary, mainly as an emergency. One patient died preoperatively, five died intra- or postoperatively. Besides the adequate valve type, prophylaxis of valve thrombosis by conscientious anticoagulation is the most important measure. Should leading symptoms - disappearance of valve click and increasing cardiac insufficiency during irregular anticoagulation - occur, immediate referral to cardiac surgery is required.