SummaryQuantitative morphometric studies were performed on the pulmonary arteries of two newborns who died at 1 and 3 days of age with both transposition of the great arteries and persistence of the fetal circulation. Similar studies were performed on two normal control subjects (age 2 and 6 days), one newborn infant (age 1 day) with secondary persistence of the fetal circulation, and one newborn infant (age 2 days) with isolated D-transposition of the great arteries. Morphometric data, summarized herein, showed greatest mean percent wall thickness in the patients with persistent fetal circulation either alone or with D-transposition of the great arteries, as compared to those without it. When wall thickness was related to external diameter of the vessel, the greatest difference in thickness was observed in the smallest vessels (<150 μ in diameter). Extension of smooth muscle peripherally and to the smallest vessels (<50 μ) was similarly most marked in those infants with persistence of the fetal circulation, either secondary or with D-transposition of the great arteries. Alevolar/artery ratio determinations were similar among the six subjects. These comparative data indicate that cyanotic congenital heart disease and persistence of the fetal circulation may coexist but at the same time be unrelated in a single patient. Furthermore when such coexistence is unrecognized, pharmacologic manipulation of the ductus arteriosus may be hazardous.