To assess the prevalence and role of the arterial duct in hearts with incomplete development of the aorticopulmonary septum, 110 autopsy specimens, comprising 100 examples of common arterial trunk and 10 cases with aorticopulmonary window, were studied. In addition to intracardiac malformations, these specimens were analyzed to determine the side of the aortic arch, the presence and location of any interruption of the arch, the presence of any anomaly of origin and course of the subclavian arteries, and the status of the arterial duct. The arterial duct was present in 33 cases, absent in 63 cases and undetermined in four cases with common arterial trunk. The duct provided an essential pathway for flow in the 20 cases with interrupted aortic arch, and in two cases with interruption of the proximal portion of the left pulmonary artery. The presence of the duct in 11 cases, in functional terms, was not essential. In the 10 hearts with aorticopulmonary window, the duct was present in eight. It was an obligatory part of the circulatory pathways in three cases where the aortic arch was interrupted. The prevalence of non-obligatory ducts was 71% in hearts with aorticopulmonary window compared to 15% in hearts with common arterial trunk. The prevalence of the duct in cases with aorticopulmonary window suggests this lesion is a later developmental defect. The functional role of an arterial duct in these hearts should be properly assessed in the clinical situation.