We used a 20-MHz, range-gated, pulsed Doppler device to measure noninvasively femoral artery blood flow velocity patterns in 24 preterm infants with patent ductus arteriosus (PDA). Mean spatial flow velocity and its temporal average were recorded by illumination of the entire flow stream. Peak systolic forward flow velocity (Vs), peak diastolic retrograde flow velocity (VD), mean flow velocity (Vf), and the peak to peak-mean velocity ratio, (VS-VD)/Vf, were determined serially in each patient. The changes in the femoral flow velocity pattern preceded or coincided with other evidence of ductal left to right shunting. Prior to PDA closure, (VS-VD)/Vf averaged 14.1 and Vf averaged 4.4 cm/sec; after ductal closure, (VS-VD)/Vf fell to 3.7 (P less than .001) and Vf rose to 9.1 cm/sec, more than twice the preclosure value (P less than .001). Large diastolic reverse flow velocity vectors and low mean velocity correlated with large left to right PDA shunts estimated from other methods of assessment. The noninvasive femoral artery Doppler approach detects patency of the ductus early in life, reflects left to right shunting in diastole, and emphasizes the impact of the PDA on systemic blood flow.