Fifty-one preterm infants (< 1500 gm) who were supported by mechanical ventilation were studied by use of serial color Doppler echocardiography to determine the hemodynamic impact of incompetence of the foramen ovale. Right and left ventricular stroke volume, measured by two-dimensional and Doppler echocardiography, were used to determine the ratio of pulmonary to systemic flow (Qp/Qs). The diameter of the color flow mapping of any interatrial shunt was measured together with pattern and velocity of that shunt. Ductal patency status was established. Most infants had some atrial shunting. The dominant direction of shunting was left to right within a bidirectional shunt pattern (75%). When the ductus was closed, there was a significant correlation between color Doppler diameter of the atrial shunt and Qp/Qs (r = 0.71). When this diameter was less than 2 mm, there was minimal impact on Qp/Qs. Measurable effects on Qp/Qs were usually seen at diameters > 3 mm when Qp/Qs ratios of up to 2:1 were recorded. Longitudinally, atrial shunting could be divided into four groups. Group 1 (n = 23) had minimal shunt or small shunts (< 3 mm) that resolved early, group 2 (n = 11) had small shunts that persisted, group 3 (n = 9) had large shunts (> 3 mm) that resolved, and group 4 (n = 6) had large shunts that persisted. Clinically there were no significant differences between the groups except that patients in groups 2 to 4 tended to having worse acute lung disease than patients in group 1 and had significantly more chronic lung disease. We conclude that many preterm infants have left-to-right atrial shunts that have a noninvasively measurable hemodynamic impact. This may have an effect on acute and chronic respiratory outcome and is likely to affect assessments of ductal shunting.