A rapid assay for determination of ribavirin in serum using solid-phase extraction (SPE), high-performance liquid chromatography (HPLC), and UV-detection was developed. The SPE uses phenylboronic acid columns with an approximately 100% recovery for ribavirin. The concentration-peak area relation was linear (r > 0.995), from 1 to 64 microM in 100 microL serum. The limit of detection was 0.1 microM. The intraassay CV was 3.2% at treatment levels (9.7 microM) and 11.5% at 0.4 microM. The method is used to monitor patients undergoing ribavirin treatment for hepatitis C (HCV). Samples from HCV-infected patients with and without renal dysfunction have been analyzed without interference of endogenous compounds. It is concluded that the method is useful for routine therapeutic drug monitoring.