The hydrogen washout technique was applied to an experimental model of microvascular repair to evaluate its potential use in determining blood flow after microvascular surgery. Three blood flow measurements were obtained in each of 10 rat hindlimbs with the hydrogen washout technique: a control, a reading with arterial flow disrupted, and a final reading after standard microvascular repair of only the saphenous artery. After repair, flow rate was 0.115 ml/minute/ml compared to 0.008 ml/minute/ml for the disrupted reading (p less than .01). The practical clinical applicability of the hydrogen washout technique for evaluating blood flow in the fingertip was tested on five human volunteers. Ischemia in the upper extremity was produced with a pneumatic tourniquet. The hydrogen washout technique and the Doppler pulse monitor were used simultaneously to evaluate circulation in the small finger. Hydrogen uptake occurred simultaneously with return of clinical signs of tissue perfusion during gradual tourniquet deflation. The Doppler pulse returned while clinical signs of ischemia remained. The use of hydrogen washout in monitoring three patients following microvascular surgery has shown it to be accurate in predicting survival. It has, thus far, proven itself to be easily repeatable and reliable both intraoperatively and postoperatively.