Therapy with interferon and ribavirin for hepatitis C virus (HCV) infection induces a decrease in several haematological population counts. It is unclear whether haematological toxicity is more severe in patients co-infected with HCV and human immunodeficiency virus (HIV). This study analysed the evolution of haematological population counts during and after interferon and ribavirin therapy for chronic HCV infection. Eleven patients co-infected with HIV and HCV and treated with pegylated interferon plus ribavirin, and ten treated with standard interferon plus ribavirin, were analysed. With reference to baseline values, neutrophil counts decreased by an average of 45% (range 18-67%), total lymphocytes by 50% (16-63%), CD4 lymphocytes by 54% (16-61%), haemoglobin by 9% (5-16%) and platelets by 31% (16-45%). The nadir of the decrease was reached in the first weeks of therapy and was maintained while patients were receiving treatment. The reduction in all series was higher with pegylated interferon. Patients recovered their baseline counts after finishing the treatment. No cases of haemorrhage or outstanding infection were detected during follow-up.