The choice between bioprosthetic or mechanical prosthetic valve replacement for active valvular endocarditis has been controversial. To establish the role of each, we reviewed the case histories of 185 patients who underwent valve replacement for active valvular endocarditis during the past 5 years. All patients had life-threatening, active bacterial endocarditis of a native or prosthetic valve. Group I (88 patients) had replacement with the Ionescu-Shiley pericardial valve and Group II (97 patients) with the St. Jude Medical valve. The male/female distribution, age range, and functional classification were the same in the two groups. Mean follow-up was approximately 20 months for both groups. Valve replacement was done because of native valve endocarditis in 76 patients in Group I and 49 patients in Group II. Of the remainder of the Group I patients, six had endocarditis of a bioprosthesis and six of a mechanical valve; of the remainder of Group II patients, 30 had endocarditis of a bioprosthesis and 18 of a mechanical valve. Early mortality was not significantly different between the two groups (14 deaths in each group). Of the 74 survivors in Group I, 15 underwent valve reoperation, 10 because of recurrent endocarditis and five because of sterile perivalvular leakage. The frequency of reoperation was significantly different (p less than 0.01) from that in Group II, in which only five patients underwent valve reoperation, four for recurrent endocarditis and one for sterile perivalvular leakage. The actuarial rate for freedom from reoperation was also significantly higher in Group II patients; 94.6% were free from reoperation at 4 years compared to 75% at 4 years in Group I patients (p less than 0.01). The actuarial survival rate, which also differed significantly between groups, was 78.7% at 4 years in Group I and 87.4% at 4 years in Group II (p less than 0.05). Patients receiving a bioprosthesis for active endocarditis had a significantly higher reoperation rate and a significantly greater incidence of recurrent endocarditis (p less than 0.01). Therefore, we prefer to use a mechanical valve for valve replacement in most patients who have active endocarditis.