Minor metabolic pathways in human subjects have been shown to exist for the conversion of codeine to hydrocodone but have not been reported for the metabolic conversion of morphine to hydromorphone. In this study, urine specimens were collected in an out-patient setting from 13 pain patients who were chronically treated with morphine and other opioids (methadone, oxycodone, and fentanyl). The chronic pain patients were chosen for study because they were treated with high-dose morphine and had no personal or family history of addiction. Results of the initial evaluation and follow up of these patients with random urine tests did not indicate opioid misuse. The specimens were analyzed by GC-MS for the presence of hydromorphone. The reporting limit for hydromorphone was 100 ng/mL. Ten of the 13 morphine-treated patients excreted hydromorphone in minor amounts ranging 120 to 1400 ng/mL. Concurrent morphine concentrations were exceedingly high in these 10 patients and frequently exceeded the upper limit of linearity (> 10,000 ng/mL) of the assay. The ratio of hydromorphone to morphine ranged from 0.015 to 0.024. Morphine concentrations in the three patients in which hydromorphone was not detected tended to be lower than those observed in other patients. For comparison, one additional patient was included in the study, who was prescribed both morphine and hydromorphone. Concentrations of hydromorphone in this patient were in the range of 3400-13,000 ng/mL, while concurrent morphine concentrations were in the range of 3200-6600 ng/mL. These data are highly suggestive that hydromorphone can be produced as a minor metabolite of morphine in humans. Although additional studies in more restricted settings are needed, it is recommended that interpretation of low urinary concentrations of hydromorphone in combination with high concentrations of morphine in morphine-treated pain patients should not be considered as conclusive evidence of hydromorphone misuse.