To study the prevalence and clinical features of hepatitis G virus (HGV)/GB virus C (GBV-C) infection in bone marrow transplantation (BMT), we examined frozen serum samples from 95 bone marrow allograft patients for HGV/GBV-C RNA by RT-PCR. Twenty-eight out of 95 (29.5%) were positive and 14 of the HGV+ patients were already positive before transplantation. The mean numbers of blood donors to whom the HGV and HGV+ populations were exposed before BMT were not significantly different (Kruskal-Wallis test, P = 0.08, NS) but did reveal that the HGV+ population had been transfused more often. Moreover, all but one of the patients who were HGV+ before graft, had had hematological diseases which needed heavy transfusion protocols suggesting, a role of blood products in HGV transmission. Fifty out of the 95 patients received Gammagard intravenous immunoglobulin (i.v.IG) batches suspected of having transmitted HCV. However, no significant difference appeared between these recipients and those receiving other i.v.IG. Despite their immunodeficiency, no clinical or biological evidence of liver disease potentially linked to HGV infection has as yet been observed. The clinical outcome, in terms of acute GVHD, chronic GVHD or veno-occlusive disease was similar in HGV+ and HGV- recipients suggesting the absence of adverse effects of HGV infection on the early outcome of allogenic BMT. Long-term evolution remains to be prospectively studied.