Standard interferon treatment is known to increase the risk of infections; this risk also needs to be evaluated in clinical practice for pegylated interferon. To this end, we studied 255 patients treated with standard (103) or pegylated (152) interferon, in combination with ribavirin, for hepatitis C. Overall, 31 anti-hepatitis C virus treatment-related infections were observed. Neutropenia (neutrophil counts below 1x10(3) cells/ml) was observed in a significantly higher proportion of patients treated with pegylated interferons (48% vs 9%; P=0.0009). Of the 31 infections, eight were respiratory infections and were observed only in patients with neutropenia. None of the non-respiratory infections was observed in patients with neutropenia. Multivariate analysis, using Cox's proportional hazards regression model, found a higher risk of all infections associated with both use of pegylated interferons [hazard ratio (HR) 4.6] and neutropenia (HR 2.46). However, neutropenia was independently associated with acute respiratory infections only and use of pegylated interferons with non-respiratory infections. In summary, use of pegylated interferon appears to increase the risk of non-respiratory infections independently from neutropenia.