Forty patients with complete atrioventricular canal (CAVC) underwent primary repair at Fukuoka Children's Hospital in Fukuoka, Japan, between August 1, 1981 and July 31, 1989. The age at repair ranged from 2 months to 6 years (mean 19 months); weight ranged from 2.3 to 22 kg. The surgical mortality was 2.5%. Justification for early primary repair was examined. Eleven patients underwent repair before 6 months of age (Group 1), 12 patients, between 7 and 11 months of age (Group 2), and 17 patients, after 12 months of age (Group 3). Degenerative changes in the atrioventricular valve increased significantly as age at repair increased (p less than 0.05 Group 1 versus Group 3). The incidence of residual mitral regurgitation tended to increase in the order of Group 1, 2 and 3, though the degree ranged from trivial to mild. Study of the left atrium/aorta ratio by echocardiography revealed that stable values of around 1.1 in Groups 1 and 2 and around 1.3 in Group 3 continued during the follow-up period of 3 years. Assessment of the diameter of the repaired mitral valve in the mean interval of 26 months in groups 1 and 2 revealed normal growth of the mitral valve annulus. The angle between the repaired mitral valve and ventricular septum, which can be affected by the growth of the ventricular septum, converged to normal range in the mean interval of 26 months. Postoperative pulmonary vascular resistance in Groups 2 and 3 was higher at 4.4 +/- 2.3 and 6.3 +/- 2.2, respectively, than in Group 1 at 3.3 +/- 2.2 (p less than 0.01 versus Group 3).(ABSTRACT TRUNCATED AT 250 WORDS)