A prospective randomized study, comparing the clinical results between two groups of patients undergoing valve replacement with the Monostrut (MS) and Medtronic-Hall (MH) heart valve prosthesis was carried out between January 1985 and December 1988. A MS valve was implanted in 205, the MH in 198 patients selected randomly. Statistical analysis showed a perfect match between the two groups regarding preoperative and operative variables, including implantation site. All patients were given life-long anticoagulant therapy after operation. Follow up was 5.06 years (median) with a total of 1971.1 patient-years (pty) and 96.6% complete. The two groups showed no significant difference in the incidence of the following postoperative events: 30-day mortality, late mortality, reoperation, anticoagulant related bleeding, paravalvular leak, prosthetic valve endocarditis and thromboembolism (TE). Multivariate analysis indicated a borderline increased hazard of TE (2.1 +/- 0.4%/pty versus 0.7 +/- 0.2%/pty) (p = 0.0087, Bonferoni corrected significance) in the MS group. It is concluded that long term outcome is excellent with these two tilting disc valves. The marginally lower incidence of thromboembolism, however, favors the Medtronic Hall prosthesis.