Six cases of ulnar nerve injury resulted from crossed K-wire fixation of displaced supracondylar humeral fractures in children. The age ranged between 4 and 10 years. Pain on extension of the little and ring fingers and early clawing were important post operative signs of ulnar nerve involvement. Early exploration of all six cases revealed medial pin placement in the cubital tunnel in five cases. In two of these, the nerve was directly penetrated, and in three, it was constricted by the cubital tunnel retinaculum. In the case 6, the nerve was hypermobile and found to be fixed anterior to its groove over the medial epicondyle. The nerve was decompressed in all cases, and the wire was repositioned. Follow-up ranged from 4 to 14 months. Full nerve recovery occurred in three cases, partial in two, and no recovery in one. Early exploration rather than simple pin removal is safer and diagnostic of the mechanism of injury.