Sarcomatoid carcinomas (SC) are a rare histologic variant of prostate cancer associated with progression in the absence of PSA elevation (1) and overall, a poor prognosis (2). These tumours commonly present alongside conventional adenocarcinoma and can show immunohistochemical (IHC) evidence of prostatic origin (3). However, loss of expression of the androgen receptor (AR) and AR-regulated proteins (including PSA and ERG in ERG gene-fusion positive cancers) in SC could introduce a diagnostic dilemma as to whether a second primary has occurred. Moreover, IHC is unable to conclusively establish whether SC and co-occurring adenocarcinomas have a common origin.This article is protected by copyright. All rights reserved.