A 63-year-old man was admitted to our hospital for complaints of a painless knot in his right testicle. The patient underwent orchiectomy for suspicion of malignancy. Pathologic examination detected a firm, circumscribed mass that consisted histologically of noncohesive, large, undifferentiated tumor cells diffusely infiltrating the testicular interstice. Because of the patient's advanced age and the structure of the neoplastic cells, the differential diagnosis favored a lymphoma over a malignant germ cell tumor but also included a dedifferentiated metastatic neoplasm. Immunohistochemistry was tailored accordingly and was completed using three successive panels of antibodies. Immunostaining with the first panel of antibodies directed against leukocyte epitopes (CD45, CD20, and CD3) remained negative and made a lymphoma unlikely. The subsequent panel (cytokeratin AE1/3, cytokeratin 18, and HMB-45) helped to rule out a malignant melanoma and aided to settle the diagnosis of a metastatic carcinoma. The reaction pattern of the last panel of antibodies pointed to a pulmonary origin of the putative primary tumor (PSA-negative, TTF-1-positive, and CK20-negative). The diagnosis of a metastasized poorly differentiated adenocarcinoma of the lung was confirmed by autopsy 5 months later. This case represents the extremely rare occurrence of a testicular metastasis as a primary manifestation of an occult neoplasm and shows the usefulness of an integrated site-specific clinicomorphologic approach that should precede and guide the choice of diagnostic immunoreagents.