A 91-year-old man developed a mass in the right medial canthal and lacrimal sac region, which was found histopathologically to be a poorly differentiated neoplasm. The results of a complete systemic evaluation, including urologic consultation, rectal examination, bone scan, and determination of serum acid phosphatase level, were normal. In spite of the negative evaluation for prostatic carcinoma, immunohistochemical studies using immunoperoxidase stains for prostaticspecific antigen demonstrated that the lacrimal sac tumor was metastatic carcinoma from an occult primary neoplasm in the prostate gland. The presence of tumor in the prostate gland was then confirmed by a needle biopsy. Modern immunohistochemical techniques are particularly valuable in the diagnosis of selected ophthalmic tumors in which the diagnosis cannot be determined by routine diagnostic techniques.