The purpose of this study is to compare the effectiveness of multiple doses of once daily tramadol HCl ER (100, 200, 300 and 400 mg) to placebo in patients with moderate to severe pain due to OA. The study hypothesis is that tramadol HCl ER is safe and effective in the treatment of patients with moderate to severe pain due to OA.

Immediate release (IR) tramadol has demonstrated efficacy in several pain conditions including: obstetrical, gynecological, orthopedic, abdominal, and oral surgery. The short elimination half-life of tramadol IR necessitates every 4-6 hour dosing to maintain optimal levels of analgesia in chronic pain. The study medication in this study is a once-daily, extended-release tramadol formulation. This is a 12-week multi-center, double-blind, randomized, dose-ranging, parallel-group, fixed-dose, placebo-controlled study. Patients with OA Functional Class I-III of the knee or hip (index joint) are eligible for participation, if appropriate criteria are met. During a 2-7 day washout period, the use of all analgesic medications will be discontinued. Eligible patients experiencing moderate to severe pain (<=40 mm on a 100 mm visual analog scale) in the index joint to be evaluated and who meet all other study criteria will enter in a 2-week,double-blind titration period. During this period, patients will be randomly assigned to receive treatment with tramadol HCl ER 100 mg, 200 mg, 300 mg, 400 mg or placebo, once a day. Patients will be titrated to their assigned dose between study days 1 - 15 and will continue at that dose for the remainder of the study (Week 12). Efficacy and safety evaluations will be collected at study visits occurring at Weeks 1, 2, 3, 6, 9 and 12 or at early termination. Study medication will be discontinued at Week 12 and patients will return after 1 week for a post-treatment visit (Week 13). Patients with unmanageable pain or with unacceptable side effects will be discontinued from the study and alternate analgesic therapy initiated, as appropriate.

Inclusion Criteria:

• Patients with American College of Rheumatology (ACR) Functional Class I-III OA of the knee or hip
• Patients with involvement of at least one knee or hip joint that warrants treatment with COX-2 inhibitors, NSAIDS, acetaminophen, or opioid analgesics for at least 75 of the 90 days preceding the screening visit
• Patients with a pain intensity score in index joint >=40 mm on the visual analog scale (VAS) at the baseline visit
• Patients who are able to discontinue acetaminophen, NSAIDS, COX-2 selective inhibitors, and other analgesics during the washout period and throughout the study
• Patients who are able to understand the study procedures and complete pain scales

Exclusion Criteria:

• Patients with a medical condition, other than OA, uncontrolled with treatment or any clinically significant condition that, in the investigator's opinion, precludes study participation or interferes with the assessment of chronic pain and other OA symptoms
• Patients with a diagnosis of inflammatory arthritis, gout, pseudo-gout or Paget's disease, that, in the investigator's opinion, interferes with the assessment of pain and other symptoms of OA
• Patients with a diagnosis of chronic pain syndrome
• Patients with an ACR or a clinical diagnosis of fibromyalgia
• Patients with a clinically significant form of joint disease or prior joint replacement surgery at the index joint
• Patients with an anticipated need for surgery or other invasive procedure in the index joint