Some studies have suggested that pulmonary hypertension in the newborn with meconium aspiration can be attributed to a primary prenatal increase in pulmonary arterial musculature; but this concept has been controversial. To examine this question, we reviewed 62 infants autopsied at The Johns Hopkins Hospital, 24 of whom demonstrated meconium aspiration, 20 with meconium staining but no aspiration, and 18 with abruptio placentae without meconium aspiration or staining. Clinical and pathologic features were evaluated and cross-sectional arterial medial area was determined at the junction of the conducting and respiratory airways in nondistended lungs. No significant difference in arterial medial area was found between infants with meconium aspiration and those with meconium staining only or abruptio placentae. In addition, circumferentially muscularized intraacinar arteries were present in all infants with meconium aspiration and abruptio placentae, and all but one infant with meconium staining alone. Comparison of lungs with and without arterial injection and fixation in distention showed that injection does not uniformly distend vessels and that formalin distention may remove or mask meconium. The study suggests that meconium aspiration and its complications, not primary structural arterial changes, account for pulmonary hypertension in infants with meconium aspiration.