To assess the effect of patent ductus arteriosus (PDA) on left ventricular output (LVO) we studied stroke volume (SV), LVO, and heart rate (HR) in 21 very low birth wt preterm neonates with clinically symptomatic PDA before and after surgical ligation. Six additional infants were also studied before PDA with left-to-right shunt was detectable by the pulsed Doppler technique. Gestational age (median and range) was 28 (24–32) wk. SV was measured by duplex Doppler and M-mode echocardiography, and LVO was calculated as product of SV and HR. LVO was 419 (305–562) mL/min/kg during symptomatic PDA. It decreased to 246 (191–292) mL/min/ kg after ligation (n = 21, p< 0.001). SV was 2.69 (1.98–4.10) mL/kg during symptomatic PDA decreasing to 1.63 (1.22–1.98) mL/kg after ductal closure (n = 21, p< 0.001). HR did not change after ductal closure. In the six infants with three examinations, LVO and SV were normal before detectable ductal left-to-right shunt and after ligation, but LVO was increased by 59.5 ± 23% (mean ± SD) (p < 0.05), and SV by 60 ± 32% (p < 0.05) during symptomatic PDA. In conclusion, preterm neonates with RDS, requiring mechanical ventilation, increased LVO during symptomatic PDA by increasing their SV, and not by changing their HR.