Summary. Ribavirin in combination with interferon alpha-2 or pegylated interferon is the standard treatment for chronic hepatitis C. The current dosage recommendations for ribavirin are based on body weight (bw). Ribavirin is mainly eliminated by the kidneys and we have recently shown that ribavirin plasma concentrations are determined primarily by renal function. It is therefore reasonable to hypothesize that side-effects of ribavirin, i.e. anaemia, should be more closely related to plasma concentrations of ribavirin than to the dose per kg bw. A total of 108 consecutive patients eligible for treatment of chronic hepatitis C were studied. Ribavirin concentrations in plasma were measured by high-performance liquid chromatography (HPLC)-UV after solid-phase extraction in trough samples taken 4, 8 and 12 weeks after the treatment commenced. A total of 213 samples were obtained and the change in the haemoglobin level and the creatinine concentration was measured in addition to ribavirin. The dose of ribavirin per kg bw did not correlate with the drop in haemoglobin level induced by ribavirin. The concentration of ribavirin was non-linearly related to the drop in the haemoglobin level as revealed by fitting a standard Hill equation type dose–response curve. The half maximal drop in haemoglobin was obtained at 4.4 μm. The results from this study suggest that the anaemia induced by ribavirin depends primarily on the concentration of ribavirin, and not on the dose per kg bw. This lends further support to the idea that ribavirin should be dosed according to renal function.