A retrospective review was performed of the results of all allograft reconstructions that had been done after the resection of an osteosarcoma or an Ewing sarcoma in a skeletally immature patient between 1982 and 1989 at The Hospital for Sick Children in Toronto. There were twenty-six patients. Six reconstructions were intercalary, sixteen were resection arthrodeses, three followed resection of a bone segment including the epiphysis (osteoarticular reconstruction), and one was a replacement of the entire humerus. Resection arthrodesis about the knee was performed with a smooth intramedullary rod and with one growth plate left intact. Six procedures were performed in the upper extremity. Excluding the patients who died, the average duration of follow-up was five years and three months. Twenty-one of the twenty-six patients had reached skeletal maturity at the time of follow-up. Eighteen (69 per cent) of the patients had a good or excellent result, four (15 per cent) had a fair result, and four had a failure. Twenty patients (77 per cent) had at least one complication (other than a limb-length discrepancy), and fourteen (54 per cent) sustained at least one fracture of the allograft. Fifteen patients who had had a reconstruction in the lower extremity had survived with survival of the allograft at the time of the latest follow-up. A limb-length discrepancy of at least two centimeters developed in nine of the fifteen patients. Five were managed with a contralateral epiphyseodesis, and one of them had an unsuccessful attempt at limb-lengthening as well. The patients who had a limb-length discrepancy of more than three centimeters at the time of follow-up had been significantly younger (p < 0.05) at the time of the reconstruction than those who had a smaller discrepancy. Three allografts (12 per cent), two of which were implanted early in the series, became infected. Soft-tissue coverage is of paramount importance for the prevention of infection, and we now routinely perform primary muscle (gastrocnemius or latissimus dorsi) transfers when dealing with an inadequate muscle envelope. Twelve patients were followed for more than four years (average, six years and seven months); they had no complications other than increased limb-length discrepancy and one subluxation of the shoulder after the first four years following the reconstruction. Although the rate of complications is higher than in adults, allograft reconstruction remains a useful option for the management of skeletally immature individuals.(ABSTRACT TRUNCATED AT 400 WORDS)