Interferon therapy of hepatitis C causes a decrease in neutrophil counts, and neutropenia is a common reason for dose adjustment or early discontinuation. However, it is unclear whether neutropenia caused by interferon is associated with an increased rate of infection. In this study, we assessed factors associated and clinical consequences of neutropenia before and during interferon therapy of chronic hepatitis C. A total of 119 patients with chronic hepatitis C treated with the combination of interferon alfa and ribavirin were analyzed. In these studies, neutropenia was not used as an exclusion or dose modification criterion. In multivariate analysis, only black race was associated with baseline neutropenia. During treatment, neutrophil counts decreased by an average of 34%. Among 3 blacks with baseline neutropenia without cirrhosis or splenomegaly, there was little or no decrease in neutrophil counts (despite typical decreases in platelet and lymphocyte counts). Documented or suspected bacterial infections developed in 22 patients (18%), but in no patient with neutropenia. United States population estimates suggest that 76,000 blacks with hepatitis C have neutrophil counts below 1,500 cells/microL and might be denied therapy if this exclusion criterion was generally applied. In conclusion, neutropenia is frequent during treatment of hepatitis C with interferon and ribavirin, but it is not usually associated with infection. Constitutional neutropenia, which is common among blacks, should not exclude patients from therapy with interferon as these patients usually have minimal further decreases in neutrophil counts on therapy and are not excessively prone to bacterial infections.