BACKGROUND Mixed cryoglobulinemia (MC) is frequently associated with clinical and biological evidence of liver disease and has recently been reported in cases of hepatitis C virus (HCV) infection. The aim of this study was to assess prospectively in a large series of MC patients: (1) the prevalence of HCV markers (anti-HCV antibodies and HCV RNA in serum and cryoprecipitate); (2) the main clinical, biologic and liver histologic features in patients with or without HCV infection. METHODS One hundred fifteen consecutive unselected MC patients were studied: 45% had well-defined underlying diseases ("nonessential" MC). Fifty-five percent with no cause of MC were considered to have "essential" MC and were subjected to in-depth examination. METHODS Patients were considered to have MC if two successive determinations of their serum cryoglobulin level were above 0.05 g/L. Anti-HCV antibodies (Ab) were detected in all patients by second-generation tests (ELISA, RIBA). We also looked for HCV RNA sequences amplified by polymerase chain reaction (PCR) in the sera and cryoprecipitates of 39 patients; HBs antigen, anti-HBs Ab and anti-HBc Ab in all patients; and HBV DNA in 20 sera and 17 cryoprecipitates. Quantitative HCV Ab and RNA studies were performed on whole serum, cryoprecipitates, and supernatants. Clinical features were recorded retrospectively for each patient. Liver biopsies from 23 anti-HCV Ab-positive and 7 anti-HCV Ab-negative patients were examined histologically, with qualitative and quantitative analysis. RESULTS Anti-HCV Ab were found in 47/115 (41%) patients by ELISA and RIBA: 33/63 (52%) essential MC and 14/52 (27%) nonessential MC. Among the 63 essential MC patients, the 33 anti-HCV Ab-positive (Group 1) were compared to the 30 anti-HCV Ab-negative patients (Group 2). Group 1 patients had more cutaneous involvement (Raynaud's phenomenon, purpura, livedo, distal ulcers, or gangrenous changes) (17 versus 5: p = 0.004), higher alanine aminotransferase levels (110 +/- 22 versus 41 +/- 10 IU; p < 0.005), higher serum cryoglobulin levels (0.35 +/- 0.07 versus 0.12 +/- 0.04 g/L; p = 0.01), lower CH50 (28 +/- 3 versus 44 +/- 2 CH50/mL; p = 0.0001) and lower C4 levels (0.20 +/- 0.02 versus 0.29 +/- 0.03 g/L; p < 0.04). The prevalence of HBV serum markers was low in both groups, and HBV DNA was never detected in any of the sera and cryoprecipitates tested. HCV RNA sequences were detected in 10/16 (63%) sera and 12/16 (75%) cryoprecipitates from Group 1 patients, whereas they were not in the sera or cryoprecipitates from 23 Group 2 patients. Using quantitative PCR, HCV RNA in cryoprecipitates was concentrated 20 to 100 times despite the absence of significant anti-HCV Ab concentration in these samples. Histologic examination of liver biopsies revealed a spectrum of lesions ranging from chronic active hepatitis to cirrhosis, but Knodell's score did not differ between the groups. CONCLUSIONS (1) About 50% of the essential MC patients had anti-HCV Ab, and these patients had more severe cryoglobulinemia-associated clinical and biological signs; (2) HCV RNA sequences were found in the large majority of sera and cryoprecipitates from patients with essential MC and anti-HCV Ab and were more concentrated in cryoprecipitates than in supernatants. These results suggest a role for HCV in the pathogenesis of MC and indicate that many cases of essential MC may be secondary to HCV infection and thus nonessential.