To evaluate the clinical course after replacement of the aortic valve with a porcine heart-valve, 87 patients (mean age 52 years [5-70] at time of surgery) were followed up for a five-year period. Operative mortality was 1% and late mortality 8%. The actuarial probability of survival at five years for all patients was 91%. Embolic events occurred in two patients and endocarditis in six. Valve dysfunctions requiring reoperation resulted from bacterial endocarditis in three patients, from primary valve dysfunction in three patients and from paravalvular leak in another three patients. Echocardiographic cross examination of the bioprostheses showed degenerative processes in eight asymptomatic patients. Patients who underwent reoperation or showed valve degeneration were significantly younger (41 +/- 19 years and 42 +/- 9 years respectively) than patients without valve degeneration (52 +/- 9 years) [p less than 0.05]. We conclude that there is an increased risk of primary valve degeneration for patients under age 35. Echo-Doppler sonographic cross examinations of the heart are a highly sensitive means of detecting early degeneration of the bioprostheses.