A rapid mass spectrometric method was developed for the identification of fentanyl and its major hepatic metabolite norfentanyl in postmortem urine of six drug-overdose victims involving fentanyl use. To reduce matrix effects or ion suppression, sample preparation consisted of centrifugation and solid-phase extraction. Deuterium-labeled internal standards ((2)H(5)-fentanyl and (2)H(5)-norfentanyl) were used to compensate for instrument variation in signal, analyte recovery during sample preparation, and ion suppression. Structural information for fentanyl and norfentanyl were collected using mass spectrometry (MS) with electrospray ionization (ESI) operated in the positive ion mode. Fentanyl (m/z 337) was found in each of the six overdose cases by the appearance of the MS-MS daughter ion on both an ion trap and a triple-quadrupole MS resulting from the fragmentation pathway of fentanyl (m/z 337 --> 188). Norfentanyl was detected in all six cases by the appearance of the MH(+) ion, m/z 233, with a single-quadrupole MS and confirmed in an ion trap MS. Ion suppression, as determined by the comparison of ion intensities from spiked samples in water with postmortem urine from the cases, ranged from 18% to 98% in three ESI sources. The use of stable isotope-labeled internal standards obviates sample preparation because ratios of analyte/internal standard remain constant in the presence of extensive matrix effects. This MS method provided sufficient sensitivity and selectivity for the rapid identification of fentanyl and norfentanyl in urine at levels >/= 10 ng/mL without prior analyte resolution by chromatography and with a total analysis time of less than 1 h.