Abstract A high-performance liquid chromatography–atmospheric pressure chemical ionization–tandem mass spectrometry (HPLC–APCI–MS/MS) reference method for the quantitation of aldosterone in serum and plasma has been developed. Samples were extracted with dichloromethane/diethyl ether, containing flumethasone as internal standard (IS). Chromatography was performed on a phenyl column using 50 m mammonium formate (pH 7.1)/methanol (50/50, v/v) as mobile phase. Analysis was in negative-ionization mode by selected reaction monitoring (aldosterone m/ z359.2 → 331.2; IS m/ z455.0 → 379.0). The assay was linear over the range 15–500 pg/mL, with limits of detection and quantitation of 10 and 15 pg/mL, respectively. Imprecisions of the assay at 15, 20, 150, and 450 pg/mL were 18.5, 8.8, 10.6, and 9.5%, respectively. The accuracy of the method ranged from 93.1 to 98.9% with absolute recoveries between 84.0 and 91.3% (aldosterone) and 88.0 and 92.3% (IS). We present a case study of a patient admitted, with suspected primary hyperaldosteronism, on the basis of a high radioimmunoassay (RIA) aldosterone concentration. The results suggest that RIA was unreliable, causing unnecessary patient discomfort and a costly 6-day hospital stay. The specific HPLC–API–MS/MS assay described offers the sensitivity and accuracy required to assess abnormal aldosterone production in hypertensive patients.