The authors report a case of metastatic prostate cancer masquerading as a primary lung tumor. Histologically, the lung tumor displayed eosinophilic crystalloids in the malignant glands typical of those previously described in prostatic adenocarcinoma. Review of histologic material from 30 additional patients with metastatic prostate cancer failed to reveal crystalloids in the metastases. Seven patients with histologic material from locally advanced prostatic adenocarcinoma, defined as spread to the rectum, bladder, or nonnodal pelvic soft tissue, were also reviewed. One of these patients demonstrated crystalloids in the bladder extension of locally advanced prostatic adenocarcinoma. Although rare, the presence of crystalloids may be used as strong evidence for the prostatic origin of an adenocarcinoma of uncertain origin.