BACKGROUND Phototherapy has been reported to alter blood flow to various tissues. We hypothesized that during phototherapy the coronary blood flow will decrease because of a 'steal effect' to the periphery. OBJECTIVES To evaluate the effects of phototherapy on coronary blood flow in healthy, jaundiced, term neonates. METHODS Flow velocity [peak diastolic velocity (Vd)] and flow measures [diastolic time velocity integral (TVId) and flow index (FI)] in the left main (LM) and left anterior descending (LAD) coronary arteries were prospectively studied with 2D/pulsed Doppler ultrasound before, during and after phototherapy in 19 healthy term, jaundiced neonates (study group) and in matched nonjaundiced controls (25 neonates). Significance was set at p < 0.05. RESULTS The neonates' characteristics were comparable in both groups. Sequential studies in the study group showed no significant decrease in measures of velocity and flow during phototherapy; however, there was a significant increase in some of these measures (Vd and TVId in the LM coronary artery, and Vd in the LAD coronary artery) after phototherapy. Velocity (Vd) and flow measures (TVId and FI) in the LAD coronary artery, but not in the LM coronary artery, were significantly lower when comparing the study group during phototherapy with the controls. In a multivariate model, phototherapy was an independent variable affecting Vd and TVId in LAD coronary arteries. Measures of cardiac output did not change significantly throughout the study. CONCLUSIONS While part of our findings may suggest a modestly lower flow in coronary arteries during phototherapy, we conclude that no clinically significant alteration in coronary arterial flow occurs during phototherapy in healthy term neonates.