We carried out extensible endoprosthetic replacement of the proximal or total humerus in 18 children aged between six and 12 years, after resection of primary bone tumours mainly for osteosarcoma and Ewing’s sarcoma. In 11 patients we performed 44 lengthening procedures, with an average of two per child annually and a mean total extension of 29.9 mm per patient. We were able to achieve lengthening of the operated limb with few complications and a mean functional rating of 79.3% according to the Enneking system. Progressive lengthening of these prostheses does not adversely affect the overall function of the arm, and superior subluxation of the head of the prosthesis has not been a problem.