A clinicopathological analysis was made of 13 cases in which the anatomical characteristics of tetralogy of Fallot were associated either with the classical features of the complete type of persistent common atrioventricular canal (8 cases) or with a forme fruste of the latter (5 cases); that is, cleft tricuspid valve (4 cases) and isolated ostium primum (one). The ratio of female to male was 10 to 3. Down's syndrome was associated in 6 of the 13 cases. Features of tetralogy of Fallot dominated the clinical picture in all cases and the diagnosis was supported by angiocardiography. The classical form of persistent common atrioventricular canal was identifiable from both the electrocardiogram and the left ventriculogram. The electrocardiogram was characteristic, showing left axis deviation with a counterclockwise loop in the frontal plane, and when available, the left ventriculogram showed the characteristic goose neck deformity. Only one patient with forme fruste of the atrioventricular canal showed this characteristic electrocardiographic picture. When the forme fruste was represented by a cleft in the tricuspid valve, the tricuspid regurgitation was not recognizable clinically. The right ventriculogram, however, presented evidence of tricuspid regurgitation.