From January 1984 to December 1993 a total of 154 patients (89 men and 85 women) required 160 reoperations for prosthetic heart valve dysfunction. Four patients required a second, two patients a third reoperation. Age was (mean +/- SD [range]) 38.8 +/- 10.2 (17 to 64) years. The primary operation was mitral valve replacement in 105 patients, aortic valve replacement in 20, and both aortic and mitral valve replacement in 29. The time interval between initial valve replacement and reoperation was 66.4 +/- 40 (3 to 288) months for the mechanical prostheses and 68.7 +/- 32 (24 to 140) months for bioprostheses; the difference was not statistically significant. Primary tissue failure was the most common cause of the reoperation for bioprostheses and valve thrombosis for mechanical prostheses. The hospital mortality rate was significantly higher in the replacement of mechanical prostheses (14/58 = 24.1% vs. 7/102 = 6.8%, p = 0.004). Low preoperative functional capacity and valve thrombosis were linked to higher mortality rates. It is discussed that monoleaflet mechanical valves yielded the highest operative mortality and that, excluding these, the risk of mortality in prosthetic valve reoperations today does not differ much from that in primary valve replacements.