['To assess the prevalence of TT virus (TMV) infection in a series of patients with chronic hepatitis C virus (HCV) infection, with or without benign (mixed cryoglobulinemia) or malignant (B-cell non-Hodgkin lymphoma (B-NHL)) lymphoproliferative disease.', 'Sixty-six HCV patients were studied, including patients with mixed cryoglobulinemia (n=30), B-NHL (n=15), and no mixed cryoglobulinemia or B-NHL (n=21). All HCV patients had increased transaminase levels and were HCV RNA positive. Patients were considered to have mixed cryoglobulinemia if two successive determinations of their serum cryoglobulin level were above 0.05 g/L. Mixed cryoglobulinemia-negative patients never had mixed cryoglobulins in their serum on multiple determinations. Subjects without HCV infection included 79 patients with histologically proven B-NHL, and 50 healthy blood donors. Serum samples were analyzed for TTV DNA by nested polymerase chain reaction, with two couples of primers in different regions of the genome, in two independent laboratories.', 'In the group of HCV-positive patients, TTV DNA was found in one of 15 (6.7%) patients with B-NHL, and in nine of 51 (17.6%, P = 0.43) of those without B-NHL. Among HCV-positive patients without B-NHL, TTV DNA was more frequently found in those with type II mixed cryoglobulinemia vasculitis than in those without it (six of 16 (37.5%) versus two of 21 (9.5%), P = 0.05). In subjects without HCV infection, TTV DNA was present in 10 of 79 (12.7%) patients with B-NHL and in seven of 50 (14.0%, P = 0.82) blood donors.'] Is transfusion-associated TT virus co-infection in patients with hepatitis C virus associated with type II mixed cryoglobulinemia but not with B-cell non-Hodgkin lymphoma?