Pain is often inadequately treated in patients with cancer. A total of 1308 outpatients with metastatic cancer from 54 treatment locations affiliated with the Eastern Cooperative Oncology Group rated the severity of their pain during the preceding week, as well as the degree of pain-related functional impairment and the degree of relief provided by analgesic drugs. Their physicians attributed the pain to various factors, described its treatment, and estimated the impact of pain on the patients' ability to function. We assessed the adequacy of prescribed analgesic drugs using guidelines developed by the World Health Organization, studied the factors that influenced whether analgesia was adequate, and determined the effects of inadequate analgesia on the patients' perception of pain relief and functional status. Sixty-seven percent of the patients (871 of 1308) reported that they had had pain or had taken analgesic drugs daily during the week preceding the study, and 36 percent (475 of 1308) had pain severe enough to impair their ability to function. Forty-two percent of those with pain (250 of the 597 patients for whom we had complete information) were not given adequate analgesic therapy. Patients seen at centers that treated predominantly minorities were three times more likely than those treated elsewhere to have inadequate pain management. A discrepancy between patient and physician in judging the severity of the patient's pain was predictive of inadequate pain management (odds ratio, 2.3). Other factors that predicted inadequate pain management included pain that physicians did not attribute to cancer (odds ratio, 1.9), better performance status (odds ratio, 1.8), age of 70 years or older (odds ratio, 2.4), and female sex (odds ratio, 1.5). Patients with less adequate analgesia reported less pain relief and greater pain-related impairment of function. Despite published guidelines for pain management, many patients with cancer have considerable pain and receive inadequate analgesia.