This article describes a rapid high-performance liquid chromatographic (HPLC) method for the measurement of the primary metabolite of oxcarbazepine. Following a simple precipitation step, 10,11,-dihydro-10-hydroxy-5H-dibenzo(b,f)azepine-5-carboxamide is quantitated (5-60 microg/mL) by analysis on an HPLC-UV system. The instrument time is less than 5 min per injection, an improvement over most published methods. The assay's limit of quantitation, linearity, imprecision, and accuracy adequately cover the therapeutic range for appropriate patient monitoring. In comparison to other published methods, this procedure would be of interest to clinical laboratories because it employs a precipitation step for sample preparation, instead of conventional yet time-consuming solid-phase extraction.