243 Background: The number of core needle biopsies performed to diagnose metastatic prostate cancer are sometimes reduced in daily clinical practice. Intraductal carcinoma of the prostate (IDC-P) is associated with adverse prognostic parameters and has recently received attention as a valuable indicator for the prediction of disease severity. Currently, the relationship between IDC-P diagnosis and the number of core prostate biopsies is unclear. In the present study, we analyzed the effect of core needle biopsy number on IDC-P diagnosis in patients with metastatic castration-sensitive prostate cancer. Methods: We retrospectively evaluated data from 150 patients diagnosed with metastatic prostate cancer at our hospital between 2002 and 2012. Percentage of the core involved with cancer and the maximum cancer occupancy were 100% in median of all the patients. Subjects were allocated to three groups according to the number of core biopsies performed: ≤5, 6–9, and ≥10. The study endpoints were the cancer-specific survival (CSS) and overall survival (OS) rates. Results: Twenty-seven (18%) patients had ≤5 core biopsies, 67 (45%) patients had 6–9 core biopsies, and 56 (37%) patients had ≥10 core biopsies. For patients who underwent ≥10 core biopsies, a significant difference on CSS was detected between with or without IDC-P ( p = 0.002). On the other hand, the difference decreased as the number of core biopsies became smaller (6–9; p = 0.322 and ≤5; p = 0.815). A similar trend was identified for the OS outcome. A significant difference on OS was also found between with or without IDC-P in patients who underwent ≥10 and 6–9 core needle biopsies ( p = 0.0002 and 0.015, respectively), but not in those who underwent ≤5 core biopsies ( p = 0.3407). IDC-P served as a stronger prognostic marker for CSS and OS than did the other factors included in the multivariate analysis. ( p = 0.0024, and p = 0.0014, respectively). Conclusions: Given the IDC-P detection and its value as a prognostic marker, we recommend the performance of ≥10 core biopsy procedures in patients diagnosed with metastatic prostate cancer. Sampling error effects the IDC-P value in smaller number of core needle biopsies.