A subgroup of patients with metastatic carcinomas of unknown origin may benefit from combination chemotherapy. The relevance of immunohistochemistry in detecting such patients was investigated. Immunohistochemical studies with a panel of antibodies were performed on the tissue specimens of 41 patients having a light-microscopic diagnosis of poorly differentiated adenocarcinoma or undifferentiated carcinoma of unknown origin, who had been treated with cisplatin- containing chemotherapy. The study aimed to answer the following questions: (a) Can the tissue type of the tumor be verified? (b) Can a primary organ site be identified? (c) Can a prognostic immunohistochemical profile be recognized? The original diagnosis had to be changed in 2 of the 41 patients, who turned out to have a malignant lymphoma and neuroblastoma, respectively. The primary site was diagnosed in a patient with prostate cancer, whereas in one case the diagnosis could be narrowed down to a neuroendocrine tumor. No certain immunohistochemical profile with prognostic significance could be identified. It was concluded that immunohistochemistry should be routinely used in cases of undifferentiated carcinoma of unknown primary origin to verify the histological diagnosis and to select the appropriate therapy.