Previous work by Serlin and colleagues [Serlin R C, Mendoza T R, Nakamura Y, Edwards K R, Cleeland C S. When is cancer pain mild, moderate, or severe? Grading pain severity by its interference with function. Pain 1995;61:277-84] established cutpoints for mild, moderate, and severe cancer pain based on the pain's level of interference with function. Recent work [Jensen M P, Smith D G, Ehde D M, Robinson L R. Pain site and the effects of amputation pain: further clarification of the meaning of mild, moderate, and severe pain. Pain 2001;91:317-22; Zelman D C, Hoffman D L, Seifeldin R, Dukes, E. Development of a metric for a day of manageable pain control: derivation of pain severity cutpoints for low back pain and osteoarthritis. Pain 2003;106(1/2):35-42]found differences in cutpoints for pain severity for different pain-related conditions. Reasons for these discrepancies may relate to the methods used to determine the cutpoints or to differences based on the type or the cause of the pain. The purposes of this study were to determine the optimal cutpoints for mild, moderate, and severe pain based on patients' ratings of average and worst pain severity, using a larger range of potential cutpoints, and to determine if those cutpoints distinguished among the three pain severity groups on several outcome measures. Results from a homogenous sample of oncology outpatients with pain from bone metastasis confirm a non-linear relationship between cancer pain severity and interference with function and also confirm that the boundary between a mild and a moderate level of cancer pain is at 4 on a 0-10 numeric rating scale. However, this analysis did not confirm the boundary between moderate and severe cancer pain previously described by Serlin and colleagues [Serlin R C, Mendoza T R, Nakamura Y, Edwards K R, Cleeland C S. When is cancer pain mild, moderate, or severe? Grading pain severity by its interference with function. Pain 1995;61:277-84]. In addition, these results were not consistent with the cutpoints that were found for back pain, phantom limb pain, pain 'in general', or osteoarthritis pain reported by Jensen and colleagues and Zelman and colleagues [Jensen M P, Smith D G, Ehde D M, Robinson L R. Pain site and the effects of amputation pain: further clarification of the meaning of mild, moderate, and severe pain. Pain 2001;91:317-22; Zelman D C, Hoffman D L, Seifeldin R, Dukes, E. Development of a metric for a day of manageable pain control: derivation of pain severity cutpoints for low back pain and osteoarthritis. Pain 2003;106(1/2):35-42]. Possible explanations for these differences are discussed, as well as implications for future research.