We analyzed risk factors in 724 patients evaluable for acute graft-versus-host disease (GVHD) and in 614 patients evaluable for chronic GVHD who had received bone marrow transplantation (BMT) from HLA-identical siblings and/or parents for thalassemia and/or microdrepanocytosis, in a single institution. The overall incidence of grade II-IV and III-IV acute GVHD (aGVHD) was 26.9% and 13.5%, respectively. The cumulative incidence of grade II-IV aGVHD in patients treated with cyclosporine (CsA)/methylprednisolone (MP) or CsA/methotrexate (MTX)/MP was 32% and 17%, respectively (P=0.001). In logistic regression analysis, the risk factors associated with the onset of grade II-IV aGVHD in the entire group of patients were: patient age < or = 4 years (P=0.009), male patient sex (P=0.023), GVHD prophylaxis with CsA/MP or MTX/MP (P=0.000), more than twofold elevated alanine aminotransferase (P=0.001), and patient seropositivity for two to three herpes viruses (P=0.007). In patients treated with CsA/MP, splenomegaly > 2 cm (P=0.042) and donor age > or = 17 years (P=0.034) predicted aGVHD. Risk factors for grade III-IV aGVHD were similar to the risk factors identified for grade II-IV aGVHD. Moreover, moderate and severe liver fibrosis or cirrhosis predicted grade III-IV aGVHD (P=0.018). The incidence of chronic GVHD (cGVHD) was 27.3%. The probability of cGVHD at 2 years after BMT in patients with grade 0, I, II, and III-IV aGVHD was 15%, 32%, 53%, and 54%, respectively. Among patients with absent or grade I-IV aGVHD, prior aGVHD (P=0.000), female donor sex (P=0.000), use of alloimmune female donors for male patients (0.009), and GVHD prophylaxis with CsA/MP or MTX/MP (P=0.003) predicted cGVHD. This data should be considered in clinical management and in future investigations for improvement of immunosuppressive prophylaxis in BMT patients with thalassemia.