Four children, who developed ulnar nerve palsy following percutaneous fixation of supracondylar fracture of the humerus by Kirschner wires (K-wires) are reported. In one the K-wire was removed within 48 h and the neuropraxia recovered immediately. In the other three patients recovery took an average of 6 weeks after removal of the wires and only after exploration of the ulnar nerve. The nerve was found to be trapped behind the medial epicondyle of the humerus. Release and subsequent transposition of the nerve resulted in recovery. Certain measures are suggested to prevent this avoidable complication in the treatment of supracondylar fractures of the humerus in children.