We successfully predicted that patients presenting with critical ischemia of a limb and Doppler ratios greater than or equal to 0.3 would benefit from lumbar sympathectomy alone. However, we found that the procedure failed in 14 per cent of limbs whose ratios were greater than or equal to 0.3. In retrospect, all these patients were found to have deep infection. Had we known this fact prospectively, our predictions for success would have been close to 100 per cent. We were less than 50 per cent successful in predicting failure of the procedure, but the number of patients in this group is too small to draw reliable conclusions. We believe that patients with arm-ankle Doppler ratios greater than or equal to 0.3 whose manifestations of ischemia are limited to the skin will have a greater than 95 per cent chance of receiving a good result from lumbar sympathectomy alone and that this result will be maintained for many years.