Hepatitis C virus (HCV) infection has been implicated in the pathogenesis of mixed cryoglobulinaemia. Several studies have shown the presence of anti-HCV antibodies and HCV-RNA in both sera and cryoglobulins of such patients. However, the prevalence and clinical significance of cryoglobulins remain uncertain in patients with chronic HCV infection. We have studied 113 consecutive patients referred for assessment because of the presence of anti-HCV antibody in serum for the presence of cryoglobulinaemia and ascertained their clinical relevance and immunochemical properties. Twenty-one of 113 (19%) had detectable cryoglobulins with a mean protein concentration of 0.38 g/l (range 0.15-3.34 g/l). Most of these patients were asymptomatic. The cryoglobulins were of type III in 19 (91%) and of type II in two patients (9%). The latter two patients had the highest concentration of cryoglobulins, subnormal C4 and C1q levels suggesting classical pathway activation and vasculitis with renal impairment. The cryoglobulin IgG subclasses were mainly IgG1 and IgG3. HCV-RNA was detected more frequently in the sera of cryoglobulin-positive patients than in cryoglobulin-negative patients. This study showed that mixed cryoglobulinaemia is common in chronic HCV infection, and is predominantly type III. Evidence of systemic or renal disease was rare except in those with type II cryoglobulinaemia, and this may reflect either the concentration of the cryoprecipitate or the presence of a monoclonal complement-activating IgM paraprotein. The detection of HCV-RNA in the majority of the cryoprecipitates further supports the important role of HCV in the etiopathogenesis of essential mixed cryoglobulinaemia, although the mechanism is at present unclear.