Morphine is present in varying amounts as an endogenous product in human urine. Derivatization of morphine contained in urine with dansyl chloride yields a known product, which can be quantified by liquid chromatography mass spectrometry with high selectivity and sensitivity. Urine samples of 51 healthy individuals were spiked with stable-isotope labeled morphine, hydrolyzed and subjected to solid phase extraction followed by derivatization of morphine with dansyl chloride. The dansyl derivatives of naturally occurring morphine and deuterated internal standard were then detected by liquid chromatography-triple quadrupole mass spectrometry. Using the [N-CD(3)]-labeled internal standard and solid-phase extraction, a limit of detection of 35 fmol/ml (10 pg/ml) and a limit of quantification of 87.5 fmol/ml (25 pg/ml) was determined for morphine in human urine. This new LC-MS/MS method allowed the detection of endogenous morphine in human urine of 51 volunteers with an average value of 156.4 fmol/ml (44.7 ng/ml).