We retrospectively analyzed the data base of the Italian Association of Pediatric Hematology/Oncology BMT Group on the incidence and severity of GVHD in children given allogeneic BMT from HLA-identical sibling and receiving cyclosporin A (CsA) alone as GVHD prophylaxis. The study population included 145 patients for acute GVHD and 114 children at risk for chronic GVHD. Twelve patients had non-malignant diseases and 133 patients were affected by malignant disorders. Among the 145 patients (50 females, 95 males), 107 (74%) presented acute GVHD and 38 (26%) had no sign of disease. In the group of patients with acute GVHD, 38 children (26% of the whole study population) were found to have grade II disease, 9 (6% of the whole) grade III, 4 (3%) grade IV. Donor-recipient sex pairs had no significant influence on incidence of acute GVHD neither did donor-recipient age class stratification. Of the 114 patients evaluated for chronic GVHD, 86 (76%) developed no disease while 23 patients (20%) presented secondary chronic GVHD and 5 (4%) had de novo chronic GVHD. The incidence of chronic GVHD was higher in F-M than in M-M donor-recipient sex pairs (33% vs 11%, p < 0.05), with no difference between F-F and M-F. In patients of > 10 years, a higher incidence of chronic GVHD was observed in both female donors and recipients compared with male donors and recipients (48% vs 20% and 47% vs 19%, respectively, p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)