Patients who are chronically infected with hepatitis C virus (HCV) often develop mixed cryoglobulinemia (MC), a B-cell proliferative disorder with polyclonal activation and autoantibody production. We investigated if MC is associated with a deficit of CD4(+)CD25(+) immunoregulatory T (Treg) cells, which have been shown to control autoimmunity. Because Treg cells express higher amounts of CD25 than activated CD4(+) T cells, we analyzed blood CD4(+)CD25(high) Treg cells in 69 untreated patients chronically infected with HCV. Treg cell frequency in patients without MC (8.8% +/- 2.3%) or with asymptomatic MC (7.4% +/- 2.1%) was comparable to that of healthy controls (7.9% +/- 1.3%). In contrast, it was significantly reduced in symptomatic MC patients (2.6% +/- 1.2%, P <.001) even when compared to a panel of untreated HCV(-) patients with different inflammatory disorders (6.2% +/- 0.8%, P <.0001). In symptomatic MC patients, the purified remaining CD4(+)CD25(+) T cells retained suppressive activity in vitro. These results, together with experimental data showing that depletion of Treg cells induces autoimmunity, suggest a major role of Treg cell deficiency in HCV-MC vasculitis and this is the first report of a quantitative Treg cell deficiency in virus-associated autoimmunity.