To determine the clinical characteristics and outcome of patients with chronic hepatitis C virus (HCV) infection presenting severe autoimmune cytopenia unrelated to interferon alpha therapy, we analyzed characteristics and outcomes of 35 patients with HCV (16 from our departments and 19 from the literature). We considered active autoimmune hemolytic anemia (AHA) as a decrease of at least 2 g/dL in hemoglobin levels, an increase of at least 0.6 mg/dL in the serum unconjugated bilirubin level, a reticulocyte count >5%, and a positive direct Coombs test. Severe neutropenia was defined as a neutrophil count <0.5 x 10(9)/L, and severe thrombocytopenia as a platelet count <30 x 10(9)/L. We identified the following cytopenias: AHA (17 cases), severe thrombocytopenia (16 cases), aplastic anemia (2 cases), severe neutropenia (1 case), refractory sideroblastic anemia (1 case), and pure red cell aplasia (1 case). Three patients simultaneously presented 2 types of severe cytopenias. Twenty-seven patients (77%) were female and 8 (23%) male, with a mean age at diagnosis of cytopenia of 51.7 years (range, 18-84 yr). Immunologic markers were detected in 19 (68%) of 28 patients, the most frequent being hypocomplementemia in 16 (57%), cryoglobulins in 15 (54%), antinuclear antibodies in 12 (43%), and rheumatoid factor in 5 (18%). Other associated processes were autoimmune diseases in 14 (50%) of 28 and human immunodeficiency virus (HIV) coinfection in 3 (9%) of 32. We found clinical and immunologic differences between HCV patients with AHA and those with severe thrombocytopenia. Patients with HCV-related AHA showed a higher prevalence of associated autoimmune diseases (71%), cryoglobulins (67%), and cirrhosis (59%). All had a good response to corticosteroids, but a poor prognosis (47% mortality). In contrast, patients with HCV-related severe thrombocytopenia had a lower prevalence of associated autoimmune diseases (11%), a poorer response to corticosteroids (55%), and lower mortality (6%), with HIV/HBV coinfections in some patients. The 35 cases presented demonstrate that different types of immune-mediated cytopenias may be severe and clinically significant in patients with HCV infection. Hemolytic anemia and severe thrombocytopenia were the most frequent cytopenias observed. Most patients responded well to corticosteroids, although a higher rate of mortality was observed in those with liver cirrhosis.