The origin of a primary or metastatic carcinoma in the pelvic area is sometimes difficult to establish, in particular the distinction between those originating in the bladder and the prostate. A candidate marker is the HOXB13 gene, essential for prostate development. Some studies have shown expression of HOXB13 protein by immunohistochemistry in the nuclear compartment of benign prostate luminal epithelium and prostate carcinoma. Forty-two cases of biopsies and resection specimens of the prostate and urinary bladder, metastatic lymph nodes, and pelvic masses were retrieved from our databases. In all cases, doubt persisted regarding prostatic versus urothelial origin. All cases were stained for CK7, p63, p504s, PSA, CK20, and HOXB13. Chromogranin A, CD56, and synaptophysin were used when neuroendocrine differentiation was suspected. HOXB13 staining was negative or only weakly positive in all carcinomas of urothelial origin. Three of four carcinomas with neuroendocrine differentiation did not express HOXB13. The fourth carcinoma, in a patient with a history of prostate carcinoma, was positive. In two cases with a synchronous prostatic and urothelial carcinoma, HOXB13 was exclusively expressed in the prostatic carcinoma. Our results demonstrate that HOXB13 expression identifies prostatic origin of a carcinoma with good sensitivity (89 %) and very good specificity (100 %). HOXB13 is a specific and sensitive marker for prostate cells and a valuable diagnostic tool, especially when poorly differentiated or neuroendocrine tumors are encountered. These results justify testing of HOXB13 as a prostate-specific carcinoma marker in larger cohorts for a more thorough evaluation of its sensitivity and specificity.