We review our experience with the immunoperoxidase technique of staining tissue for prostate-specific antigen in four patients with atypical metastases from prostate cancer. Our results indicate that this test is clinically useful for the diagnosis of metastatic prostate cancer in patients with an unsuspected primary prostate malignancy. Further, application of prostatic-specific antigen testing may confirm metastatic prostate cancer in atypical sites in patients with a previously diagnosed prostate malignancy.