This was a randomized, open-label, comparative, parallel group study designed to demonstrate the noninferiority of once-daily OROS^® hydromorphone compared with twice-daily sustained-release (SR) oxycodone in subjects with chronic noncancer pain severe enough to require continuous opioid therapy. The core phase (24 weeks) consisted of titration and maintenance periods. This was followed by an optional extension phase (28 weeks), which collected data used to assess long-term safety and efficacy outcomes. Five hundred four subjects were randomized between the 2 treatment groups. The primary efficacy analysis showed that OROS hydromorphone was noninferior to SR oxycodone (P=0.011) as measured by change in Brief Pain Inventory (BPI) pain severity subscore "pain right now." The treatment difference with respect to change in BPI pain severity subscore "pain right now" was 0.29 (95% confidence interval: -0.27 to 0.84). The equianalgesic doses were 16mg OROS hydromorphone and 40mg SR oxycodone (median values). Secondary outcomes included other BPI scale items, the Medical Outcomes Study (MOS) Sleep Indices, and quality of life measured by the Short Form 36 (SF-36) questionnaire. Both treatment groups showed improvements in the main secondary efficacy endpoints. No statistically significant differences were shown between the treatment groups, except for the scores for somnolence (MOS sleep subscale) and physical functioning (SF-36), which both had a statistically significant difference between treatments groups in favor of OROS hydromorphone. Both study medications had equivalent and acceptable safety profiles. The results of this open-label study showed that once-daily OROS hydromorphone is a safe and well-tolerated treatment for chronic pain and as efficacious as twice-daily SR oxycodone.