Thirty three per cent of children with obstetrical brachial plexus palsy with incomplete neurological recovery develop shoulder internal contracture associated with osseous deformity. Some of the older children are treated by humeral derotational osteotomy. The classical technique of open approach to the humeral diaphysis and plate fixation imposes a longitudinal scar and carries significant risks (nonunion, nerve palsy); a secondary procedure for plate removal is necessary in a significant proportion of patients. The authors report a new technique of percutaneous humeral osteotomy with osteosynthesis by Hoffmann external fixator. In six cases bone healing was obtained at an average of 45 days, without adverse complication. The postoperative results showed improved shoulder function. This new technique is simple and safe; it represents a new option for the treatment of sequelae of obstetrical brachial plexus palsy.