Aims Intraductal carcinoma (IDC) is an adverse histopathological parameter for prostate cancer outcome, but is not incorporated in current tumour grading. To account for its dismal prognosis and to omit basal cell immunohistochemistry, it has been proposed to grade IDC on the basis of its underlying architectural pattern. The aim of this study was to determine the impact of IDC grade assignment on prostate cancer biopsy and radical prostatectomy tumour grading. Methods and results A cohort of 1031 prostate cancer biopsies and 835 radical prostatectomies were assigned a Grade Group according to the 2014 International Society of Urological Pathology guidelines, without incorporation of IDC in grading. Tumour grading was compared with a Grade Group in which IDC was graded on the basis of its underlying architecture. Of 1031 biopsies, 139 (13.5%) showed IDC. Grade assignment of IDC led to a Grade Group change in 17 (1.6%) cases: four of 486 (0.8%) Grade Group 1 cases were reclassified as Grade Group 2, nine of 375 (2.4%) Grade Group 2 cases were reclassified as Grade Group 3, and four of 58 (6.9%) Grade Group 4 cases were reclassified as Grade Group 5. IDC was observed in 213 of 835 (25.5%) radical prostatectomies, and its grading led to a change in tumour grade in five of 835 (0.6%) patients, with upgrading in two of 207 (1.0%) patients with Grade Group 1 cancer, in two of 420 (0.5%) patients with Grade Group 2 cancer, and in one of 50 (2%) patients with Grade Group 4 cancer. Conclusion IDC grade assignment led to a Grade Group change in 1.6% of prostate biopsy specimens and in 0.6% of radical prostatectomy specimens. Although the inclusion of IDC in or the exclusion of IDC from the Grade Group might affect decision‐making in individual patients, it has a minimal impact on overall prostate cancer management.