We studied 14 premature infants with the clinical diagnosis of peripheral pulmonic stenosis (PPS) and 15 normal full-term neonates by echocardiographic Doppler examinations. The PPS group had an average main pulmonary artery (PA) diameter similar to the control group (0.91 vs 0.96 cm, difference not significant), but had smaller branch PA diameters: right PA = 0.41 vs 0.50 cm, p less than 0.001, and left PA = 0.41 vs 0.49 cm, p less than 0.001. The PPS group also had greater peak velocities in the main PA (76 vs 63 cm/s, p less than 0.05), right PA (193 vs 118 cm/s, p less than 0.001) and left PA (187 vs 123 cm/s, p less than 0.001). Similarly, the ratio of peak velocity in the branch/main PA was greater for the PPS group: right/main PA peak velocity = 2.91 vs 1.92, p less than 0.01, and left/main PA peak velocity = 2.73 vs 1.99, p less than 0.05. The calculated right ventricular output for the PPS group was more than the control group: 437 vs 261 ml/min/kg, p less than 0.001. Hematocrits were not done on the control group, but the PPS group had an average hematocrit which was low (34%). It is concluded that patients with PPS have mild underdevelopment of the PA branches, with consequent increased flow velocity and turbulent flow. This turbulent flow may be contributed to by increased cardiac output and mild anemia.