In 343 patients treated surgically for mitral valve disease, there was thrombosis of the left atrium in 40. Massive atrial thrombosis was present in 32 of them, yet the diagnosis was suspected or established preoperatively in only 12, because of insufficient clinical signs. A transventricular mitral commissurotomy was performed in 32 patients and mitral valve replacement in the remaining 8. Surgical mortality was 23% in the patients with atrial thrombosis compared with 7% in the trombus-free patients: 66% of the deaths were due to cerebral embolism. Systemic embolism occurred in 17% of the patients who had a closed approach initially and in 27% following a planned open procedure. Among the survivors, 65% were asymptomatic or markedly improved; the average follow-up was 42 months. Atrial thrombosis increases the surgical risk because of the high incidence of operative embolism, a complication that is not avoided by the use of open procedures, but that could be prevented by earlier surgical treatment of mitral stenosis, before atrial thrombi develop.