Arterial thromboembolic complications were studied in 196 patients who had either a single Björk-Shiley or Lillehei-Kaster aortic disc valve implanted. Eight patients suffered from such complications in the course of the first postoperative month and three of them died, two from myocardial infarction and one from cerebral embolism. Nineteen late thromboembolic complications developed in 18 of the 164 patients who survived the postoperative period, the incidence bein 5.9 episodes per 100 patients per year. The two valve types wer found to be equallly thrombogenic, and the rate was not lower than that in patients with Starr-Edwards aortic ball valves of series 2,300 previously studied. Particularly serious was valve malfunction caused by thrombi that limited the movement of the discs. Early recognition of this condition is essential, because the only effective therapy is removal of the thrombus. Three patients with a Björk-Shiley and one with a Lillehei-Kaster valve suffered this complecation and two died, while cerebral embolism caused a third late death. Two of the three patients who had not received anticoagulants developed thromboembolic complications, while most episodes occurred in spite of well-maintained anti-coagulant treatment. It is concluded that arterial thromboembolic complications remain a considerable problem also after aortic disc valve implantation, and that thrombotic valve malfunction is particularly serious and requires special attention.