A stable isotope dilution liquid chromatography-electrospray ionization tandem mass spectrometry (LC-MS/MS) assay to measure amiodarone, the most frequently used agent for maintaining sinus rhythm in patients with atrial fibrillation, and its major metabolite desethylamiodarone in human plasma and serum was developed. Measurement of amiodarone and desethylamiodarone was performed during a 4.0-min run-time using amiodarone-D(4) and desethylamiodarone-D(4) as internal standards. Calibration curves covering 12 calibrators measured in four replicates each for the analysis of both amiodarone and desethylamiodarone were linear and reproducible in the range of 0.01-40.0 mg/L (r>0.999). Limits of detection in plasma matrix were 2.7 microg/L for amiodarone and 1.9 microg/L for desethylamiodarone, and lower limits of quantification in plasma matrix were 7.5 microg/L for amiodarone and 2.5 microg/L for desethylamiodarone. Interassay imprecision and inaccuracy were <8% and <9% for both substances. Mean extraction yield was 99.6% (range 92.6-107.7%) for amiodarone and 90.2% (range 80.0-94.7%) for desethylamiodarone. Agreement was moderate for amiodarone (n=162) and desethylamiodarone (n=117), respectively, between the present method and a HPLC method with UV detection using a commercially available reagent set for the HPLC analysis of these drugs. The Passing-Bablok regression line was HPLC=0.98 (LC-MS/MS)+0.10 [mg/L]; r=0.94 for amiodarone and HPLC=1.05 (LC-MS/MS)+0.02 [mg/L]; r=0.90 for desethylamiodarone. This sensitive and interference-free LC-MS/MS assay permits rapid and accurate determination of amiodarone and desethylamiodarone in human plasma and other body fluids.