Doppler-derived blood flow velocity measurements were used to characterize the hemodynamics of 66 internal mammary artery grafts and 60 saphenous vein grafts to the coronary arteries at operation. Pulsed Doppler spectral analysis of centerstream graft flow demonstrated predominantly diastolic flow with a variable, multiphasic flow pattern in systole. The magnitude and configuration of the graft flow velocity waveform varied with graft type and whether the runoff was to single or multiple arteries. At operation, peak diastolic flow velocity was greater (P less than 0.0001) in internal mammary artery grafts to a single outflow artery (71 +/- 2 cm/sec) compared with single vein grafts (31 +/- 4 cm/sec). Sequential grafts demonstrated increased flow velocity and forward flow throughout the pulse cycle, indicative of low outflow resistance. Analysis of the phasic flow patterns permitted an assessment of functional graft patency. Technical errors (anastomotic stricture, internal mammary pedicle torsion) were identified in three grafts with low or absent diastolic flow. Vasospasm of the internal mammary artery was associated with high flow velocity throughout the pulse cycle. Observed differences in patency and the development of intimal hyperplasia between internal mammary artery and saphenous vein grafts may be related to graft hemodynamics.