Metastatic tumors of unknown origin. The clinical appearance of metastatic lesions without an obvious primary source for the tumor is a common event. Tumor Registry figures and epidemiologic data grossly understate the actual frequency of unknown primaries, because primary sites are often "assigned" to patients on a best-guess basis without positive proof of a tumor's origin. In the majority of patients whose primary tumors have continued to elude detection, the extensive use of diagnostic imaging studies fails to produce information that alters the patients' clinical course. Rare exceptions will be cited, but these exceptions prove the general rule. Imaging studies should therefore be targeted for selected patients with disseminated malignancies in whom identification of their primary tumors could benefit quality of life or length of survival. 