Mucoid adenocarcinoma of the prostate is a very rare variant, account less than 1% of prostatic adenocarcinomas. In this respect, the most common histopathological type of prostate cancer is acinar adenocarcinoma. Diagnosis of this variant is very important due to peculiarities: aggressive biologic behavior, poor response to radiotherapy. Although these tumors are not as hormonally responsive as acinar adenocarcinomas, some of them respond to androgen withdrawal. Before making a diagnosis of primary mucinous adenocarcinoma, is necessary to exclude an extraprostatic malignant neoplasia, especially from the bladder or bowel. We present the case of a male patient who suffered a transurethral prostatic resection surgery. Histopathological examination revealed malignant tumor cells floating in a pool of mucus or even acini mixed with signet ring cells. A particular aspect is that floating mucus cells have a non-papillary pattern similar to colloid carcinoma of the breast. The use of monoclonal antibody revealed positive immunoreaction of malignant cells for prostatic specific markers and excluded neoplastic invasion of the bladder or bowel carcinoma.