(Objective) We investigated the prognostic significance of intraductal carcinoma of the prostate (IDC-P) in radical prostatectomy specimens. (Materials and methods) We evaluated 441 patients treated with radical prostatectomy and analyzed data on IDC-P, lymph node metastases, Gleason score, seminal vesicle invasion, extraprostatic extension, surgical margin, total cancer volume, and zonal origin of dominant cancer focus in radical prostatectomy specimens. The median follow-up was 50 months (range 6-164 months). (Results) We identified IDC-P in 112 cases (25.4%). The five-year biochemical progression-free survival rate in patients with IDC-P was significantly lower than for those without IDC-P (35.8% vs 69.6%; p<0.0001). In a univariate analysis, IDC-P (p<0.0001), lymph node metastases (p=0.0022), Gleason score (p<0.0001), seminal vesicle invasion (p<0.0001), extraprostatic extension (p<0.0001), surgical margin (p<0.0001) and total cancer volume (p<0.0001) were significantly associated with the biochemical progression-free survival. In a multivariate analysis, Gleason score (p<0.0001), IDC-P (p=0.0002), seminal vesicle invasion (p=0.0011), extraprostatic extension (p=0.0012), surgical margin (p=0.0019) and lymph node metastases (p=0.0402) were significantly associated with biochemical progression-free survival. (Conclusions) The presence of IDC-P is an independent factor of biochemical recurrence in prostate cancer patients treated with radical prostatectomy. We therefore recommend that the presence of IDC-P in radical prostatectomy specimens be reported.