Background Bone metastases are a common painful and debilitating consequence of castration-resistant prostate cancer (CPRC). Bone pain may predict patients' prognosis and there is a need to further explore CRPC patients' experiences of bone pain in the overall context of disease pathology. Due to the subjective nature of pain, assessments of pain severity, onset and progression are reliant on patient assessment. Patient reported outcome (PRO) measures, therefore, are commonly used as key endpoints for evaluating the efficacy of CRPC treatments. Evidence of the content validity of leading PRO measures of pain severity used in CRPC clinical trials is, however, limited. Methods To document patients' experience of CRPC symptoms including pain, and their impact on health-related quality of life (HRQL), semi-structured in-depth qualitative interviews were conducted with 17 patients with CRPC and bone metastases. The content validity of the Present Pain Intensity (PPI) scale from the McGill Pain Questionnaire (MPQ), and the 'Average Pain' and 'Worst Pain' items of the Brief Pain Inventory Short-Form (BPI-SF) was also assessed. Results Patients with CRPC and bone metastases present with a constellation of symptoms that can have a profound effect on HRQL. For patients in this study, bone pain was the most prominent and debilitating symptom associated with their condition. Bone pain was chronic and, despite being generally well-managed by analgesic medication, instances of breakthrough cancer pain (BTcP) were common. Cognitive debriefing of the selected PRO measures of pain severity highlighted difficulties among patients in understanding the verbal response scale (VRS) of the MPQ PPI scale. There were also some inconsistencies in the way in which the BPI-SF 'Average Pain' item was interpreted by patients. In contrast, the BPI-SF 'Worst Pain' item was well understood and interpreted consistently among patients. Conclusions Study findings support the importance of PRO measures of pain severity as key endpoints for evaluating the efficacy of treatments for CRPC, particularly for patients with bone metastases where episodes of BTcP are common. Qualitative evidence from CRPC patients supports the content validity of the BPI-SF ''Worst Pain' item and promotes use of this item for measuring pain severity in this population.