Itraconazole has been shown to be highly effective against a broad spectrum of fungi, particularly aspergillus. In order to achieve the desired pharmacological effect, the itraconazole plasma concentration should be > 250 ng/ml. We developed a high-performance liquid chromatography assay for the rapid determination of plasma itraconazole concentration using solid-phase extraction and small sample volume. The assay possessed linearity up to 5,000 ng/ml, sensitivity of 50 ng/ml, average recovery of 100.8%, and run-to-run precision (n = 10) for concentrations of 100, 200, and 500 ng/ml of 12, 6.6, and 5.8%, respectively. Furthermore, the assay proved to be free of interference from 48 commonly prescribed and some over-the-counter medications. We conclude that the method described here is ideally suited for the therapeutic monitoring of itraconazole.