A selective and sensitive HPLC method is described for therapeutic drug monitoring of the antidepressant drug mirtazapine and its active metabolite desmethylmirtazapine. Liquid/solid extraction with C18 cartridges was used for cleanup of plasma samples. The chromatographic separation was carried out on a phenylhexyl column. No interference from other coadministered antidepressants has been observed in 234 samples from 184 patients. The calibration range used was from 1 ng/mL to 100 ng/mL. The analytic method has proven robust and well suited for therapeutic drug monitoring. In addition to qualitative and quantitative validation data for the assay method, concentration measurements in samples from patients on mirtazapine therapy and the relevant dosing information are presented. Median drug levels after a 15-mg dose were 37 ng/mL mirtazapine and 20 ng/mL desmethylmirtazapine. When a 60-mg dose was administered, median concentrations of 83 ng/mL mirtazapine and 65 ng/mL desmethylmirtazapine were found.