Forty-three infants with common atrioventricular canal (CAVC) underwent repair before 24 months of age at Children's Hospital Medical Center, Boston, between Jan. 1, 1975, and June 30, 1980. From 1975 to 1977, the operative mortality was 62% (8/13); the late mortality was 7% (1/13). From 1978 to 1980, the operative mortality was 17% (5/30) and the late mortality, was 6% (2/30). Seventeen (63%) unselected patients of the 27 long-term survivors have undergone cardiac catheterization 10 to 19 months following the operation. Five (29%) had mitral regurgitation of either moderate (four) or severe (one) degree. One had a significant residual shunt. The pulmonary/systemic resistance ratio (Rp/Rs) remained the same or decreased after repair except in the patient with severe mitral regurgitation. Anatomic features which determine outcome of reparative procedures are (1) deficiency of atrioventricular valve tissue, (2) the presence of ventricular hypoplasia, (3) malalignment of the common atrioventricular valve (CAVV) with respect to the ventricles, (4) the presence of double-orifice mitral valve, (5) the presence of solitary left ventricular papillary muscle group, and, in our experience, (6) the presence of additional muscular ventricular septal defects (VSDs). Four of these six anatomic variables can now be evaluated preoperatively by axial angiography and subxiphoid two-dimensional echocardiography. However, the architecture of the CAVV is extremely variable, so that the development of a "uniform approach" to valve reconstruction is impossible; mitral dysfunction is likely to remain the major cause of operative mortality and long-term morbidity.