Allogeneic peripheral blood stem cell transplantation (PBSCT) was performed in children and adolescents for the treatment of malignant (n = 49) and nonmalignant hematological disease (n = 8). Granulocyte colony-stimulating factor (G-CSF)-mobilized PBSCs were apheresed from 57 HLA-matched siblings aged 9 months to 24 years (median, 8 years) without any serious adverse, effects. No abnormalities were found in these donors for a median follow-up of 25 months (range, 6-56 months). Patients were conditioned with a TBI-containing regimen (n = 17) or a non-TBI regimen (n = 40). GVHD prophylaxis consisted of methotrexate (MTX) plus cyclosporine A (CSP) for 23 patients, CSP plus methylprednisolone (mPDN) for 22 patients, MTX only for 7 patients, CSP only for 4 patients, and MTX plus CSP plus mPDN for 1 patient. Engraftment was prompt, with a median number of days to reach an absolute neutrophil count (ANC) above 0.5 x 10(9)/L of 13 days (range, 8-23 days), with 1 graft failure. Acute GVHD (grades II-IV) occurred in 8 (16%) of 49 evaluable patients, and chronic GVHD developed in 23 (64%) of 36 evaluable patients. Notably, two thirds of chronic GVHD was extensive. The Kaplan-Meier estimate of 3-year disease-free survival was 0% for refractory disease (n = 6), 37.2% +/- 11.8% for high-risk malignancies (n = 25), 81.4% +/- 9.7% for standard-risk malignancies (n = 18), and 100% for nonmalignant disease (n = 8). The estimated 100-day nonrelapse mortality rate was 9.9% +/- 4.2%. In conclusion, allogeneic PBSCT is feasible in a pediatric population. Although the grade of acute GVHD was set low, as in Japanese BMT studies, the incidence and severity of chronic GVHD appears to be relatively high. For nonmalignant disease, the question arises of whether the higher incidence and severity of chronic GVHD is a drawback of this procedure. For high-risk malignancies, whether or not a graft-versus-leukemia effect prevents relapse needs to be clarified in future comparative studies with BMT.