Recognition is growing that self-report of drug use, prescribed or otherwise, among patients with chronic pain treated with opioids is often unreliable. This fact is well known to the addiction management community. Patients may inaccurately report use of prescribed medications, fail to report use of nonprescribed medications or medications prescribed by other physicians, or fail to report use of illicit drugs. Although there are yet no accepted diagnostic criteria for addiction or other forms of medication misuse in the patient with chronic pain, most clinicians would agree that awareness of a patient's inappropriate use of nonprescribed medications or illicit drugs is relevant to proper patient management. The use of external sources of information, therefore, such as testing of biologic material (e.g., urine), interviews with spouses, review of medical records, or input from prescription monitoring programs, may improve patient management. Of these methods, urine toxicology testing has by far the largest experience. Urine toxicology testing may reveal the presence of illicit drugs, such as heroin or cocaine, or controlled substances not prescribed by the physician ordering the test (e.g., hydromorphone in a patient prescribed oxycodone). The authors review the use of urine toxicology testing in monitoring patients with chronic pain, including laboratory aspects. They also present evidence from recent studies that suggests that monitoring the behavior alone of patients on chronic opioid treatment will fail to detect potential problems revealed by urine toxicology testing. The authors conclude that, although further research is urgently needed, at this time it is appropriate to conduct routine urine toxicology testing in patients with chronic pain treated with opioids.