The European Association of Palliative Care recommends the Brief Pain Inventory questionnaire (BPI) as a pain assessment tool in clinical studies. After translation into Norwegian, we administered the BPI to 300 hospitalized cancer patients. Cronbach's alphas were computed to assess reliability, and factor analysis was utilized to ascertain construct validity. The BPI interference and pain severity scales were validated against items on pain intensity and pain influence on daily function in the European Organization for Research and Therapy of Cancer (EORTC) QLQ-C30 questionnaire. In total, 235 patients (78%) were able to complete the BPI questionnaire, but 82 (35%) of these questionnaires had one or more missing items. Cronbach's alphas were 0.87 for the pain severity and 0.92 for the interference scales. A factor analysis identified three factors; pain intensity, interference with physical function, and interference with psychological functions/sleep. These three factors explained 82% of the variance. The correlation between BPI pain severity index and the EORTC QLQ-C30 item on pain intensity was 0.70 (P < 0.001). The correlation between BPI interference index and the EORTC QLQ-C30 item on pain influence on daily living was 0.62 (P < 0.001). We conclude that BPI has satisfactory psychometric properties, but is not completed by a significant proportion of patients. Further research is needed to establish pain assessment tools for patients unable to answer a comprehensive pain questionnaire, to establish routines for analysis of missing values, and to investigate if pain interference items also reflect disease-related impairment.