Now that systemic-pulmonary collateral arteries are used for unifocalization in patients with pulmonary atresia and ventricular septal defect, the question arises whether morphologic criteria of these collateral arteries could help to provide better results. In an attempt to classify the morphologic features of systemic-pulmonary collateral arteries, we studied 31 heart-lung autopsy specimens with pulmonary atresia and ventricular septal defect. The course of the systemic-pulmonary collateral arteries (origin, branching pattern, and connections with systemic and central pulmonary arteries) was related to their histologic characteristics. The results show that systemic-pulmonary collateral arteries cannot be classified according to their course related to the trachea and the main branches of the bronchi. The histologic features of these collateral arteries vary along their course to the lungs. Nearly all systemic-pulmonary collateral arteries contain a muscular or a musculoelastic segment. One type of collateral artery (complex loop anastomoses) is completely muscular and resembles a bronchial artery. Nutritive branches (bronchial arteries) arise from all histologic types of systemic-pulmonary collateral artery segments. The size and number of intimal proliferations in muscular, elastic, and musculoelastic segments did not differ significantly. In 29 of 31 cases a ductus arteriosus did not coexist with large collateral arteries (two cases unknown). It is concluded that a classification of large systemic-pulmonary collateral arteries based on morphologic features results in a highly variable system, which does not facilitate decisions for the suitability of these arteries for unifocalization procedures. The variability of the systemic-pulmonary collateral arteries corresponds with the recent embryologic finding that during development, collateral artery formation is possible during extended periods.