Sweating was induced in 56 patients for the purpose of predicting the results of lumbar sympathectomy. Nitrazine paper was applied to the skin as an indicator of the appearance of perspiration. The limb affected by vascular disease was then compared with the contralateral limb. A prediction formulated on the basis of this test was recorded before each of 68 sympathectomies performed in these patients, and the findings after operation were then compared. Among 41 nondiabetic patients, the good results that had been predicted in 39 cases, and the poor results predicted in two, were realized. Among 15 diabetic patients, however, there were five prediction-failures. If a patient with ischemic extremities is free from diabetes, the sweating test should help in planning the treatment.