Purpose: The objective of our study was to determine the optimal cut points for classification of pain scores as mild, moderate, and severe based on interference with function and quality of life (QOL). Methods: We evaluated 822 patients who completed the Brief Pain Inventory (BPI) and/or the European Organization for Research and Treatment of Cancer (EORTC) QOL Questionnaire Core 30 (QLQ-C30) prior to receiving repeat radiation therapy for previously irradiated painful bone metastases. Optimal cut points for mild, moderate, and severe pain were determined by the MANOVA that yielded the largest F ratio for the between category effect on the seven interference items of BPI and the six functional domains of QOL (physical, role, emotional, cognitive, social functioning, and global QOL) as indicated by Pillai’s Trace, Wilk’s λ, and Hostelling’s Trace F statistics. Results: For BPI and for QOL domains separately, the two largest F ratios for Wilk’s λ, Pillai’s Trace, and Hotelling’s Trace F statistics were from the cut points 4, 8 and 6, 8. When combining both, the optimal cut points were 4, 8 with 1–4 (mild), 5–8 (moderate), and 9–10 (severe). With this classification, the mean scores of all the seven interference items in BPI and the six functional domains were all highly statistically different. Patients with severe pain survived significantly shorter than those with mild and moderate pain (p < 0.0001). Conclusion: Our analysis supports the classification of pain scores as follows: 1–4 as mild pain, 5–8 as moderate pain, and 9–10 as severe pain. This may facilitate conduct of future clinical trials.