In this study we investigated and quantified the effects of interscalene block (ISB) on humeral arterial blood flow (HBF). Eleven patients scheduled for shoulder arthroscopic surgery under ISB were prospectively studied. A Doppler ultrasound of the humeral artery was performed before, and 30 min after, the ISB. The resistance index and the HBF were measured at the level of the midpoint of the upper arm. The median (interquartile range) of resistance index decreased from 0.98 (0.95–1.00) to 0.81 (0.77–0.91) (P < 0.01). The median HBF increased from 32 (18–46) to 88 (59–98) mL/min (P < 0.01). We conclude that ISB enhances arterial blood flow and decreases arterial resistance.