Twenty-one specimens of classically corrected transposition have been studied in order to elucidate the morphology of the pulmonary outflow tract and the nature and origin of structures which obstruct it. The tract is an oblique channel wedged deeply between the inverted mitral and tricuspid valves. As a consequence of septal malalignment, the pulmonary valve ring overrides the muscular ventricular septum. In most specimens, the left wall of the tract is patent owing to incomplete formation of the interventricular portion of the membranous septum. This permits communication of the pulmonary artery with the left-sided morphologic right ventricle. Obstruction to the pulmonary outflow tract may be valvular or muscular or may result from the presence of fibrous tissue tags. Obstruction or atresia was present in 12 specimens (57 per cent). Fibrous tags were the most common cause, and these originated from the intact or perforated membranous septum, the inverted tricuspid valve, or the pulmonary valve. The unusual anterior relationship of the cardiac conducting tissue to the outflow tract in the anomaly is emphasized.