Hepatitis C virus (HCV) infection is associated with most mixed cryoglobulinemia (MC) syndromes. In this study, HCV RNA was detected in the peripheral blood mononuclear cells of 11 (73.3%) of 15 patients with MC and in 5 (71.4%) of 7 noncryoglobulinemic patients with chronic hepatitis type C. All patients with cryoglobulinemia and 3 (42.8%) of the 7 without cryoglobulinemia (P < .05) had HCV RNA in bone marrow cells. Subjects in both groups with HCV-positive bone marrow also had HCV RNA in serum. The majority of patients with MC syndromes were infected with HCV subtypes 1b and 2a. Two patients with MC had different genotypes in serum and cells. Further studies are needed to determine which bone marrow cell population is preferentially infected by HCV and to determine if this phenomenon is involved in inducing the production of cryoglobulins.