We performed a prospective study in 186 patients with a minimum follow-up of two years in order to attempt to predict successful major re-implantation of the upper limb. There were 137 men and 49 women with 24 amputations of the palm, 75 of the wrist, 50 of the forearm, 9 disarticulations through the elbow, and 28 amputations through the upper arm. The degree of injury to the amputated segment and the stump were good predictors of the rate of success and the final outcome. Adequate preservation, contraction of the muscle in the amputated part after stimulation, the level of injury and a smoking habit were fair indicators, but the serum potassium concentration in the amputated segment was the best objective predictor. When it is higher than 6.5 mmol/l 30 minutes after re-perfusion, re-implantation should be avoided. A high systemic venous serum potassium concentration was also found before clinical signs of the re-perfusion syndrome were seen.