The muscular ventricular septal defect associated with the atrioventricular canal is a malformation which has not yet been extensively studied. Between June 1982 and December 1989, 151 patients with atrioventricular canal underwent echocardiography and angiocardiography in our Department. Of these 95 (62.9%) had a complete form and 56 (37.1%) a partial. Among the 151 patients, 81 (53.6%) presented Down syndrome. We found 5 muscular ventricular septal defects in 4 patients; in 3 cases there was a single defect and in one case two defects. These defects were midmuscular in all patients and one patient also presented an apical defect. All 4 patients with muscular ventricular septal defect presented a complete atrioventricular canal and aortic coarctation; 3 out of 4 patients had a hypoplastic left ventricle with absence of Down syndrome. The muscular ventricular septal defect is a malformation which is rarely associated with atrioventricular canal (4/151 = 2.6%). In our experience, it was always associated with a complete form with aortic coarctation and was very rare in Down syndrome patients (1/81 = 1.2%). These findings may represent a peculiar association of anomalies which may be caused by fetal hemodynamic mechanisms.