An increasing number of long-term surviving bone marrow transplantation (BMT) recipients have recovered from their primary disease but are at risk of developing failure of endocrine organs. We investigated 30 recipients who underwent allogeneic BMT during childhood or adolescence. Testicular growth and function were evaluated by serial measurement of testicular volume, basal luteinizing hormone (LH), basal follicle-stimulating hormone (FSH) and testosterone levels and by gonadotropin-releasing hormone (GnRH) provocative test. Puberty started spontaneously in all patients. However, all except four patients had normal testosterone levels with elevated LH, indicating partial Leydig cell dysfunction. Standard deviation scores of testicular volume at last evaluation were statistically lower in those who had received irradiation without gonadal shield compared to those with (-2.04+/-0.45 vs -0.30+/-1.17, respectively, P<0.005), suggesting damage of testicular germinal epithelium owing to gonadal irradiation. Serial measurement of testicular volume showed a tendency of growth to stop at 10 ml in those without gonadal shield. Among the 30 patients, only one patient has fathered a child after reaching spontaneous puberty. These results suggest that gonadal shield is effective to protect testicular growth and function, although the attainment of fertility is difficult to achieve.