Six cases of single papillary muscle of the left ventricle in persistent common atrioventricular canal (AVC) are described. Except for one case with double outlet right ventricle and AVC, in the other five cases AVC occurred with normally related great vessels. Down's syndrome was present in two cases. The mitral valvular condition in AVC is not the classic parachute mitral valve since the separation between the common anterior and the common posterior leaflet leaves a space (cleft) oriented toward the ventricular septum. This mitral valve in AVC may or may not be obstructive. The common anterior leaflet fixed to the ventricular septum may result in obstruction. Clinically, single left ventricular papillary muscle should be suspected if the AVC is associated with coarctation of the aorta or tubular hypoplasia of the aortic arch. This type of mitral valve in AVC may also be associated with double outlet right ventricle. Preoperative recognition of the single papillary muscle by echocardiogram or left ventriculogram will prevent postoperative mitral obstruction following closure of the mitral cleft.