The oncocytic variant of prostatic adenocarcinoma is exceptionally rare with only 4 cases reported in the English literature.Little is known about the clinical behavior of this variant of prostatic adenocarcinoma,because of the exceptionally low number of reported cases.The 2016 World Health Organization Classification of Tumors of Prostate does not recognize the oncocytic variant,again likely related to the exceptional paucity of reported cases.Here,we report the fifth case of the oncocytic variant of acinar type prostatic adenocarcinoma in an asymptomatic 64-year-old Caucasian American male with elevated serum prostate specific antigen(7.33 ng/m L;normal range 0-4.00 ng/mL) during routine blood screening for diabetes mellitus.At subsequent transrectal prostate biopsy,the right side of prostate was infiltrated by adenocarcinoma with tumor cells forming variably differentiated glands,including some poorly differentiated.Tumor cell nuclear:cytoplasmic ratio was low,with small to intermediate sized vesicular nuclei and only rare discernable small nucleoli.Cellular cytoplasm was characteristically granular pink with sharply defined cell membranes.Positive AMACR(P504S) epithelial immunohistochemical staining and absence of staining for prostatic basal cells confirmed the tumor to be primary prostatic adenocarcinoma.AMACR immunohistochemical staining was also helpful with accurate grading of the tumor due to the difficulty of differentiating tumor cells from residual prostate myocytes at routine hematoxylin and eosin(HE) staining.This new case adds to the exceptionally small number of previously reported cases of the oncocytic variant of primary prostatic adenocarcinoma.It also highlights the difficulty associated with Gleason scoring of the oncocytic variant by routine HE evaluation and the usefulness of AMACR(P504S) immunostaining for accurate grading of prostatic adenocarcinoma in the oncocytic variant. The oncocytic variant of prostatic adenocarcinoma is exceptionally rare with only 4 cases reported in the English literature. Little is known about the clinical behavior of this variant of prostatic adenocarcinoma, because of the exceptionally low number of reported cases. The 2016 World Health Organization Classification of Tumors of Prostate does not recognize the oncocytic variant, again likely related to the exceptional paucity of reported cases. Here, we report the fifth case of the oncocytic variant of acinar type prostatic adenocarcinoma in an asymptomatic 64-year-old Caucasian American male with elevated serum prostate specific antigen (7.33 ng / m L; normal range 0-4.00 ng / mL) during routine blood screening for diabetes mellitus. At subsequent transrectal prostate biopsy, the right side of prostate was infiltrated by adenocarcinoma with tumor cells forming variably differentiated glands , including some poorly differentiated. Tumor cell nuclear: cytoplasmic ratio was low, with small to interme diate sized vesicular nuclei and only rare discernable small nucleoli. Cellular cytoplasm was characteristically granular pink with sharply defined cell membranes. Positive AMACR (P504S) epithelial immunohistochemical staining and absence of staining for prostatic basal cells confirmed the tumor to be primary prostatic adenocarcinoma. AMACR immunohistochemical staining was also helpful with accurate grading of the tumor due to the difficulty of differentiating tumor cells from residual prostate myocytes at routine hematoxylin and eosin (HE) staining. This new case adds to the exceptionally small number of previously reported cases of the oncocytic variant of primary prostatic adenocarcinoma. It also highlights the difficulty associated with Gleason scoring of the oncocytic variant by routine HE evaluation and usefulness of AMACR (P504S) immunostaining for accurate grading of prostatic adenocarcinoma in the oncocytic variant.