The object of this study was to evaluate the metabolism of oxycodone to oxymorphone in a pain patient population using a quantitative liquid chromatography-tandem mass spectrometry analysis of 32,656 urine specimens obtained from pain patients between March 2008 and Feb 2010. The observed excretion was modeled using logarithmic transformation and approximated a Gaussian distribution. Oxycodone excretion into urine had a geometric mean of 1.93 mg/g of creatinine and oxymorphone had a value of 0.41 mg/g of creatinine. Increasing concentrations of oxycodone correlated with a smaller proportion of oxymorphone excretion suggesting saturation of oxycodone metabolism. Urine samples containing oxycodone without oxymorphone allowed an estimation of the proportion of poor metabolizers (2.4 ± 2.1%) in the population. A similar analysis of samples containing oxymorphone without oxycodone gave an estimate of the proportion of ultra-rapid metabolizers (1.8 ± 1.1%) in the population. Samples with concentrations of oxycodone above 10 mg/g of creatinine showed a sub-population of subjects with metabolic ratios roughly 100-fold less than the linear predictive model in this study. This study describes typical ranges for oxycodone and oxymorphone in urine, and showed that it is possible to identify fast or slow metabolizers who may be at risk for adverse events.