Sedation and tiredness are among the most frequent symptoms among cancer patients. A detailed assessment of these symptoms is necessary to evaluate therapeutic effects, such as the use of methylphenidate or comparison of different opioids. The Brief Fatigue Inventory (BFI) has been validated as a short and comprehensive instrument to assess severity of fatigue and fatigue-related impairment in cancer patients. We validated the German version of the BFI in patients with chronic cancer-related and noncancer-related pain treated in a tertiary pain center. Patients treated in the Pain Clinic of the Department of Anesthesiology completed the BFI, the minimal documentation system (MIDOS) and the short form SF-36 quality-of-life questionnaire (SF-36). Test-retest reliability was assessed with a second BFI immediately after the consultation and in a subgroup of patients after 3 to 7 days. Nineteen percent of the 117 patients were treated for cancer-related pain (C); the other patients suffered from chronic severe pain of nonmalignant origin (NC). Patients reported mean values for average fatigue of 3.9 (C) and 4.9 (NC), and for worst fatigue of 5.5 (C) and 6.2 (NC). The mean score of the 6 impairment items was 4.3 in both groups. Factor analysis led to a solution with one common factor for all nine items. Fatigue on the BFI correlated highly with 'feeling tired' in the SF-36 and with 'sedation' in MIDOS, and less with 'being worn out' in SF-36 and 'weakness' in MIDOS. Internal consistency was high, as was test-retest reliability, with a correlation of the intensity, mean scores of 0.93 and the impairment mean scores of 0.87. In conclusion, we found the German version of the BFI to be reliable and valid for cancer and noncancer patients. Minor differences were seen in the validation compared to the original version.