Hemodynamic measurements were used to establish predictive criteria for determining success of lumbar sympathectomy. Blood flow was measured by strain-gauge plethysmography before and after α-blockade and after sympathectomy in 40 patients. Ankle systolic pressure and ankle-brachial pressure index were determined in these and in an additional 44 cases. Results: α-blockade offers no predictive value to the effect of sympathectomy; 60% of patients had either false-positive or false-negative results. Level of ankle systolic pressure and pressure index correlated significantly with effect of sympathetic ablation. Twenty-six of 27 patients with a pressure index <0.205 required subsequent amputation. All patients with a pressure index >0.35 responded well to lumbar sympathectomy. Conclusion: Response to sympathetic ablation depends upon adequacy of collateral circulation, which is easily assessed by the Doppler ultrasound method.