Cruciferous vegetables contain glucosinolates, which are degraded to isothiocyanates. These are easily absorbed, conjugated to glutathione, and excreted into the urine as their corresponding mercapturic acids. We have developed and validated a solid phase extraction-high-performance liquid chromatography-electrospray ionization mass spectrometry/mass spectrometry method for the specific analysis of individual isothiocyanate mercapturic acids in urine. The range of reliable analysis was 1.0-310 microM in urine. Urine samples fortified with three different levels of isothiocyanate mercapturic acids were measured on six different days by three independent technicians. The relative standard deviation (RSD) of repeatability was 12, 6, and 3%; the RSD of reproducibility was 19, 14, and 8%, and spike recoveries were 103, 104, and 103%, respectively, for 1.04, 10.5, and 313 microM levels. In 24 h urine collected from two volunteers after they consumed broccoli and cauliflower, clearly sulforaphane mercapturic acid (133 micromol) and allyl isothiocyanate mercapturic acid (4.7 micromol) were found. This procedure demonstrates a reliable and efficient method to study the intake and mode of action of isothiocyanates in animal studies and clinical trials.