Sixty-four children have had a single aortic valve replacement under 16 years of age, 50 for rheumatic disease (47) or bacterial endocarditis (3) (group I) and 14 for a congenital aortic valve lesion (group II), 38 were disk prostheses and 26 were ball prostheses. Associated procedures had to be performed 31 times, with widening of a small aortic annulus by a patch in 7 patients. The early mortality was 12.5%. Of 56 survivors, 55 were followed postoperatively for a mean period of 7 years (group I: 44, group II: 11). Forty of the 55 patients were anticoagulated (correctly maintained in only 24 patients), 15 were not anticoagulated. A high rate of late complications was observed. Thrombo-embolic accidents in 5 patients with inefficient anticoagulant treatment, 2 haemorrhagic episodes, 7 prosthetic leaks; specific problems related to this group of young patients were: recurrence of rheumatic fever with increasing severity of mitral valve disease requiring mitral valve replacement in 5 patients and outgrowth of the prosthesis, which affected 7 patients; this complication is the result of either fibrous deposit around the valve annulus or such a small annulus that the surgeon could only implant a small prosthesis. Ten patients required 11 reoperations for various reasons. The main reason for reoperation was mitral valve replacement for worsening of mitral valve disease caused by recurrence of rheumatic fever. A high late mortality 10/55 (18%) was noted. The main cause of death was a perivalvular leak (5); 1 late death was caused by a stenotic number 17 Björk-Shiley valve.(ABSTRACT TRUNCATED AT 250 WORDS)