It's a multicenter prospective study in France. All children under 18 years are included, just before the HSCT. The functional test before the HSCT at 6, 12, 18, 24, 30 and 36 months will be collected and a cardiopulmonary exercise test is done one and three years after the HSCT. Thoracic tomodensitometry is done before the HCT and after at 6, 12, 24 and 36 months.