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A sensitive, simple, and fast liquid chromatography–tandem mass spectrometry (LC–MS/MS) bioanalytical method was developed to determine remifentanil in human plasma and prevent its instability by esterases during quantification. A 20 μL sample of human plasma, acidified with formic acid, was precipitated using 100 μL of acetonitrile. Chromatography was carried out on an Agilent Poroshell 120 EC-C18 column (4.6 × 50 mm, 2.7 μm) with an isocratic elution by acetonitrile and distilled water containing 0.1% formic acid (65:35, v/v) at a flow rate of 0.5 mL/min. Transition ions were detected as m/z 377.10→113.20 and m/z 277.00→111.00 for remifentanil and chlorpropamide (internal standard), respectively, with positive electrospray ionization in mass spectrometry. The run time was only 3 min per sample. We evaluated the selectivity, linearity, carry-over, accuracy, precision, extraction recovery, absolute matrix effect, stability, and incurred sample reanalysis and found that all these parameters were within acceptable limits. The calibration curve range for remifentanil was 0.05–50 ng/mL with regression coefficient (r) values higher than 0.9939. Given the simple and fast sample preparation and the lower LLOQ concentration compared to those in other methods, this method was successfully used to quantify plasma levels after intravenous infusion of remifentanil to intensive-care-unit patients during extracorporeal membrane oxygenation. [ABSTRACT FROM AUTHOR]