Pulmonary artery (PA) calcification is unusual in children, reportedly occurring only in conjunction with severe underlying valvular disease. Three newborns were found by chest X-ray study to have thin calcified rings of the PA. Two were pump twins in twin reversed arterial perfusion (TRAP) sequences, each with an acardiac cotwin, and the third was the recipient in a twin transfusion syndrome (TTS). Both twin pairs with TRAP sequence were premature, each pump twin presenting with cardiac decompensation. One remained well at 5 months of age, the other died at 5 1/2 months of age. The third, the premature recipient of twins with TTS, died at 24 h of age, and the donor cotwin died at 2 h of age. The two hearts were structurally normal at autopsy. Subtle intimal wrinkling was seen above the pulmonary valve leaflets, and the media was yellow and granular on cut section. Hyperplasia of the intima and media with disruption and calcification of the elastic fibers of the media was seen microscopically. These three cases of PA calcification occurring in utero were not related to structural valvular abnormalities but were presumably due to increased cardiac output in utero as each occurred in the volume-overloaded twin of the twin pair. Comparison of the weight differences between these three twin pairs with PA calcification suggests a relationship between the presence of PA calcification and the severity of the cardiac volume overload. In utero cardiac damage may contribute to the high morbidity and mortality rate seen in infants with TRAP sequence and TTS.