We have analyzed the presence of hepatitis C virus (HCV) and hepatitis G virus (HGV) sequences in bone marrow and serum samples from 48 patients of a hematologic outpatient clinic. HCV RNA was detected in 18 (38%) and 15 (31%) and HGV RNA was detected in 6 (13%) and 9 (19%) of serum and bone marrow samples, respectively. In 3 patients, HGV RNA was detectable in bone marrow but not in the serum; 2 of these patients were negative for the presence of specific antibodies. Using a highly strand-specific Tth-based reverse transcriptase-polymerase chain reaction (RT-PCR), the presence of HCV RNA and HGV RNA negative strand was demonstrated in 4 and 5 bone marrow samples, respectively. Our study shows that HCV and HGV can replicate in bone marrow; in the case of HGV, analysis of serum may underestimate the true prevalence of infection. (Blood. 2000;95:3986-3989)