The authors investigated 63 consecutive patients (average age 6 years 6 months) who underwent cross-fixation with three Kirschner wires after reduction of a completely displaced supracondylar fracture (type 3) of the humerus. All fractures were reduced and fixed by inserting two parallel Kirschner wires in the lateral side, followed by one crossed medial Kirschner wire under fluoroscopic guidance. Lateral pins were inserted in parallel or divergent fashion to ensure stability. With a medial crossed pin insertion, the elbow was carefully extended for easy palpation and protection of the ulnar nerve without displacing the reduced fracture. Skin incision for detection of the ulnar nerve before medial Kirschner wire fixation was not required. There was no iatrogenic ulnar nerve injury caused by the Kirschner wires. The clinical outcome of the surgery after an average of 17 months was investigated: 62 (98.4%) of the 63 patients studied showed a "satisfactory" result. Cross-fixation with three Kirschner wires is considered an effective and safe method for avoiding ulnar nerve injury in the treatment of a completely displaced supracondylar fracture of the humerus in children.