Purpose: The purpose of this study was to examine attitudinal barriers to cancer pain management and their relationship to pain, analgesic use, clinical, and demographic variables, as well as QOL, in a large sample of patients receiving strong opioids for pain in three European countries. Methods: Participants in the present study were 555 patients with cancer, 18 years and older recruited from six centers in Germany, Iceland, and Norway. All had received strong opioids for at least 72 h. Data was collected with the Barriers Questionnaire-II, the Brief Pain Inventory, and the European Organization for Research and Treatment of Cancer QLQ-C30. Results: The mean (SD) age of patients was 61.68 (12.35) years and 53% were men. Most common diagnoses were gastrointestinal, lung, prostate, and breast cancer. The mean (SD) time from diagnosis was 32.24 (44.55) and 4.97 (9.64) months from start of opioid therapy. Mean (SD) pain severity was 3.19 (1.93) on a 0 to 10 scale, and 46.5% reported worst pain of 7 or higher. Attitudinal barriers had a mean (SD) of 1.95 (0.82) on a 0–5 scale, with fear of addiction as the strongest barrier across countries 2.85 (1.49). Barrier scores increased with age, and were higher among men than women. Higher barrier scores were associated with higher pain severity and interference, and lower performance status, but not with global health-QOL. Patients who had been on opioids for a shorter time reported higher barriers. Conclusions: Attitudinal barriers are frequent in cancer pain patients on opioids and are associated with less effective pain control.