A high-performance liquid-chromatographic method is reported for monitoring theophylline in plasma. Samples are deproteinized with 2.5 volumes of acetonitrile and the supernates chromatographed on a reversed-phase column. Absorption at 275 nm is monitored. One can accurately measure 1.5 mg of theophylline per liter, in as little as 10 microliter of plasma, and only about 7 min is required per sample. No interference was found in plasma samples from asthmatic patients. This method is particularly useful for routine therapeutic monitoring of both pediatric and adult patients.