Phenethyl isothiocyanate (PEITC) is a dietary compound present in cruciferous vegetables that has cancer-preventive properties. Our objective was to develop and validate a novel liquid chromatography-tandem mass spectrometry procedure to analyze PEITC concentrations in human plasma and urine. Following hexane extraction, ammonia was added to samples to derivatize PEITC to phenethylthiourea. Chromatographic separation was achieved on a C(18) column with acetonitrile/5 mM formic acid (60:40, v/v) as the mobile phase followed by tandem mass spectrometry detection in multiple reaction monitoring mode. Deuterium-labeled PEITC was used as the internal standard. The detection limit was 2 nM and calibration curves were linear from 7.8 to 2000 nM. The intra- and inter-day coefficients of variation were less than 5 and 10%, respectively. The intra- and inter-day accuracies ranged from 101.0 to 104.2% and from 102.8 to 118.6%, respectively. The recovery from spiked human plasma and urine ranged from 100.3 to 113.5% and from 98.3 to 103.9%, respectively. The assay was used to measure PEITC in plasma and urine samples obtained from subjects after consumption of 100g of watercress. This novel assay represents the first analytical method with the sensitivity and specificity to determine plasma and urine concentrations of PEITC.