We studied 51 preterm infants (< 1500 gm) with serial color Doppler echocardiography to determine the impact of incompetence of the foramen ovale on the hemodynamic implications of shunting through a patent ductus arteriosus. Doppler and two-dimensional echocardiographic measures included left atrial/aortic root ratio, right (RVSV) and left ventricular stroke volumes (LVSV), and outputs to determine relative ventricular outputs (RVSV/LVSV) and to calculate the pulmonary/systemic flow ratio (Qp/Qs), the diameter of the color flow Doppler mapping of interatrial and ductal shunts, pulsed Doppler pattern, and velocity of those shunts. The dominant direction of shunting at the ductal and atrial levels was left to right. In studies with minimal atrial shunting, there was a weak but significant correlation between RVSV/LVSV (1/(Qp/Qs)) and the left atrial/aortic root ratio, LVSV, and output index, but there was a close correlation with the diameter of the color flow Doppler of the shunt within the ductus (r = -0.8). With this diameter used as a constant, increasing color flow Doppler diameter of atrial shunt significantly reduced LVSV and increased RVSV/LVSV (1/(Qp/QS)). In infants with large ductal and atrial shunts, right ventricular output was often greater than left ventricular output. We conclude that atrial shunting has a significant impact on the hemodynamic implications of ductal shunting in many very preterm infants. This renders use of the relative ventricular outputs to calculate Qp/Qs inaccurate as a single measure of shunt size in patent ductus arteriosus. If the shunt is predominantly left to right, the most accurate assessment is provided by color flow ductal shunt diameter.