Eleven biopsies of bone regenerate during bone lengthening following the Ilizarov technique were studied. They concerned 10 patients with a mean age of 13.5 years. The length of the biopsied regenerate was between 14 and 90 mm, and the delay after the initial procedure was 23 to 502 days. The aim of this study was to get informations on the histology of the regenerate, to quantify osseous data and to propose a surgical strategy. These data were obtained by analyze of non decalcified bone, biopsies were included in a methylmetacrylate block. Measure of the mineralisation speed was determined after a double intake of tetracycline. The bony trabeculations were formed very early, as soon as the third week. They were placed like a thread, that means in an anastomotic system which can quickly become resistant. But is was necessary to keep a lengthening speed under 1 mm per day 0.7 to 0.8 mm per day seems to be the best speed in adolescents. The ossification was of membranous type, without cartilaginous stage. Osteoblastic and osteoclastic activities were extremely rich, even after one year or after the bone healing. The main point was the maturity of the regenerate acquired as soon as the 4th month.