Hepatitis C virus (HCV) has been proposed as a possible causative agent of chronic thrombocytopenia. We investigated HCV infection in a series of 51 unselected Spanish patients with chronic acquired thrombocytopenia. Anti-HCV and HCV viraemia were detected in 13/51 (22·5%) of cases; this prevalence was particularly significant when compared with HCV seropositivity in age-matched controls (0·4%). Anti-HCV-positive patients, four men and nine women with a median age of 59·3 years (range 36–72), had a mean platelet count of 55·8 × 109/l (range 12–96). Only one of our HCV-positive thrombocytopenic patients had hypersplenism. Platelet-associated IgG (PAIgG) was negative, as measured by immunofluorescent flow cytometric analysis in 11/13 HCV-positive thrombocytopenic patients. Thus, thrombocytopenia in our HCV-positive patients appeared to be non-autoimmune mediated. In six patients, a trial of recombinant α2b-interferon (IFN-α) given at a dose of 3 MU three times per week for 6–24 months gave a durable (> 1 year) and significant increase in platelet count in all six patients. The maximum increase occurred after 6 months of IFN-α therapy. In conclusion, the ability of IFN-α to increase platelet counts in HCV-positive thrombocytopenic patients supports mechanisms involving a direct role for HCV inhibiting platelet production.