Hydrocodone in combination with acetaminophen is commonly used to control moderate pain and is metabolized by cytochrome P4502D6 to form the active metabolite, hydromorphone. The purpose of this study was to determine the metabolic relationship and variability between hydrocodone and its conversion to hydromorphone using urinary excretion data from chronic pain patients. Liquid chromatography-tandem mass spectrometry was used to quantitate hydrocodone and hydromorphone concentrations in urine specimens. The first visits of 25,200 subjects who took hydrocodone and not hydromorphone and had measurable concentrations were included in this study. The geometric mean (95% confidence index) of hydrocodone and hydromorphone urine concentrations were 1.39 (1.37-1.41) mg per gram of creatinine and 0.224 (0.221-0.227) mg per gram of creatinine, respectively. The log of creatinine-corrected hydromorphone versus the log of creatinine-corrected hydrocodone showed a positive relationship (R(2) = 0.20), with 60-fold variability between subjects. The plot of the log of the metabolic ratio ([hydromorphone] divided by [hydrocodone]) versus the log of creatinine-corrected hydrocodone had a coefficient of determination of R(2) = 0.42, with 125-fold variability between subjects. Ultra-rapid metabolizers represented 0.6% of the population, whereas 4% were poor metabolizers. Within-subject variability for the excretion of hydrocodone in urine was 23-fold, whereas between-subject variability was 134-fold. Hydrocodone and hydromorphone urine concentrations showed great variability within and between subjects.