The prevalence of antibodies to hepatitis C virus (HCVAb) was investigated in 52 unselected patients with mixed cryoglobulinemia and in 84 patients with other systemic immunologic diseases. HCVAb were detected by an enzyme-linked immunosorbent assay, and their specificity was evaluated by a recombinant-based immunoblot assay. The presence of HBV-related markers was investigated in the same samples. HCVAb were found in 54% of mixed cryoglobulinemia patients, and the finding was confirmed by recombinant-based immunoblot assay in all cases. HCVAb and/or HBV markers were present in 70% of the patients. HCVAb seropositivity was significantly more frequent in mixed cryoglobulinemia patients with biopsy-proven liver involvement (P < 0.01) and with increased serum transaminase levels (p < 0.01). HCVAb were virtually absent in control patients with other immunologic diseases. These results support the notion that viral agents, i.e., HCV and possibly HBV, have a role in the pathogenesis of mixed cryoglobulinemia patients.