Platelet survival time (SURV) has correlated with thromboembolism in patients with prosthetic cardiac valves. Sulfinpyrazone increases SURV. SURV (autologous labeling with 51Chromium) was measured in 126 patients who had aortic or mitral valve replacement. These patients were followed prospectively. Ninety-four with shortened SURV received sulfinpyrazone; 32 with normal SURV were not treated with platelet suppressants. Eighty-seven patients were anticoagulated with warfarin 67 with shortened SURV and 20 with normal SURV. Eleven patients have had thromboembolism, and all had shortened SURV (2.4 ± 0.08 days; average half-time ± SEM; normal 3.7 ± 0.03 days; n = 26) none had an increase of SURV with sulfinpyrazone (2.3 ± 0.09 days). Of 83 patients with shortened SURV who did not have embolism, sulfinpyrazone increased SURV in 59 (71%) (2.6 ± 0.05 to 2.9 ± 0.06 days). Of 35 patients with shortened SURV who failed to increase SURV with sulfinpyrazone, 11 (31%) had embolism; none of 59 (0%) with an increase of SURV with sulfinpyrazone had thromboembolism. These results suggest that patients with thromboembolism after prosthetic cardiac valve replacement have shortened SURV and that patients treated with sulfinpyrazone who have thromboembolism do not have an increased SURV.