Patients with chronic low back pain on stable opioid treatment will be converted to oxymorphone extended release (ER)and enter an open-label treatment phase.During the Open-Label Titration Period (up to 28 days), patients will receive daily oxymorphone ER PO q12h. Patients stabilized on a dose that provides adequate pain relief will be randomized to either continue on the stabilized dose of oxymorphone ER or receive placebo in a double-blind fashion for a total duration of 12 weeks.