Purpose Previous studies have determined optimal cut points (CPs) for the classification of pain severity as mild, moderate, or severe using only the Brief Pain Inventory (BPI) or the BPI in conjunction with a quality of life (QOL) tool. The purpose of our study was to determine the optimal CPs based on correlation with only QOL outcomes. Methods We conducted an analysis of 298 patients treated with radiation therapy for painful bone metastases on a phase III randomized trial. Prior to treatment, patients provided their worst pain score on a scale of 0 (no pain) to 10 (worst possible pain), as well as completed the European Organization of Cancer Research and Treatment (EORTC) QOL Questionnaire Bone Metastases module (QLQ-BM22) and the EORTC QOL Questionnaire Core-15 Palliative (QLQ-C15-PAL). Optimal CPs were determined to be those that yielded the largest F ratio for the between category effect on each subscale of the QLQ-BM22 and QLQ-C15-PAL using the multivariate analysis of variance (MANOVA). Results The two largest F ratios for Wilk’s λ , Pillai’s Trace, and Hotelling’s Trace were for CPs 5,6 and 5,7. Combining both, the optimal CPs to differentiate between mild, moderate, and severe pain were 5 and 7. Pain scores of 1–5, 6, and 7–10 were classified as mild, moderate, and severe, respectively. Patients with severe pain experienced greater functional interference and poorer QOL when compared to those with mild pain. Conclusion Our results suggest that, based on the impact of pain on QOL measures, pain scores should be classified as follows: 1–5 as mild pain, 6 as moderate pain, and 7–10 as severe pain. Optimal CPs vary depending on the type of outcome measurement used.