Ribavirin (RBV) in combination with pegylated interferon is the current standard treatment for chronic hepatitis C. Exposure to RBV seems crucial for achieving the best virologic response. However, RBV may cause anemia in a dose-dependent manner. Therefore, monitoring RBV plasma levels could be useful for individual tailoring of RBV dosing. A rapid assay was developed to quantify RBV using high-performance liquid chromatography and ultraviolet detection. Extraction of RBV from plasma was performed using a novel method based on ultrafiltration in one step that allows direct injection into the high-performance liquid chromatography without any prior steps of dryness or reconstitution. The method was validated over the range of 0.05 to 5.0 μg/mL following the EMEA Validation of Analytical Procedures (CPMP/ICH/281/95) recommendations. The clinical interest of this assay was evaluated in a subset of patients under RBV therapy. Mean RBV plasma concentrations at steady state were higher in responders compared with nonresponders (3-3.2 μg/mL versus 2.2-2.3 μg/mL).