OBJECTIVES: Previous studies in cancer patients have found scores of ≥5 on 11-point pain scales to indicate pain that has a significant impact on patients’ lives. This study sought to confirm the adequacy of a ≥5 cutpoint on the BPI-SF ‘worst pain’ item for defining pain progression in CRPC patients using data collected as part of a multinational phase III clinical trial. METHODS: Patients with a BPI-SF worst pain score ≥5 were compared with patients with a score <5 in terms of Functional Assessment of Cancer Therapy – Prostate (FACT-P) subscales and total score and EQ-5D item scores. Exploratory analyses were also conducted to investigate any potential differences within designated regional subgroups of patients. All analyses were performed using treatment-blinded data collected at the first post-baseline trial visit including the above assessments (Week 12). RESULTS: A total of 464 patients completed the BPI-SF at W12 (<5 n= 411, ≥5 n= 53). Mean FACT-P Total scores for patients with a BPI-SF worst pain score ≥5 were 24.5 points lower than for patients with a score <5 (91.1 vs 115.6, p<0.0001), indicating poorer well-being. Patients with BPI-SF worst pain scores ≥5 consistently had lower scores for all FACT-P subscales (p<0.0001) except for social well-being. The magnitude of these differences, for all scales, was considerably greater than reported thresholds for meaningful difference. Results for EQ-5D item scores were in a similar direction with significantly greater impairment reported in patients with a BPI-SF worst pain score ≥5 compared with patients with a score <5 (p<0.0001). Exploratory analyses also revealed similar results across all regional subgroups of patients. CONCLUSIONS: Patient scores ≥5 on the BPI-SF ‘worst pain’ item are associated with significant and meaningful impairments in CRPC patients, thus supporting the adequacy of this cutpoint as an appropriate definition of pain progression in this population.