Our surgical experience with 77 patients having atrioventricularis communis has been reviewed. Twenty-two of the patients had the complete form and 55, the incomplete. The anatomic variations of this lesion have been described with particular reference to the mitral valve. Operative mortality for the complete form was 63 per cent and for the incomplete, 25 per cent. Three late deaths have occurred three months to three years following operation. The mortality rate has been related mainly to heart block, pulmonary hypertension, associated lesions and to the complexity of malformations encountered in some of the cases. The operative technic employed in this group of patients has been described with particular regard to the problems involved in the correction of mitral insufficiency. Long term follow-up by heart recatheterization has demonstrated hemodynamic, improvement in the majority of patients. Residual mitral insufficiency, however, demonstrated by selective left ventriculography, was still common (in 3 of 7 patients) and residual shunts (by oximetry) in 4 of the 24 patients undergoing right heart catheterizations. The preoperative electrocardiograms in 74 patients were typical for A-V canal and atypical in 3. Postoperatively, P-R interval shortening and shift of the frontal plane QRS axis to the right has occurred in the majority of the patients. In 3 of the latter, normal QRS axis was recorded following operation.