Unknown primary cancer (UPC) is defined by the presence of metastatic disease for which a primary site is undetectable on presentation. Computed tomography scan of the body was performed routinely in search of the primary cancer and invasive procedures were pursued in selective cases. Magnetic resonance imaging of the breast enables identification of an occult breast primary tumor in ≤ 75% of women who present with adenocarcinoma in the axillary lymph nodes and can influence surgical management. Positron emission tomography scan also can be used in the diagnosis of UPCs, but its value is controversial. Cytokeratins 7 and 20 and thyroid transcription factor are some of the histochemical markers used in most patients who present with metastatic adenocarcinoma. Some of the newly discovered immunohistochemical markers further assist in narrowing the differential diagnosis. The role of molecular profiling to make the diagnosis, establish the prognosis, and assess the response to treatment in UPCs is evolving. The authors discuss the role of histochemical markers in the diagnosis of UPC and the most recent data regarding the use of imaging and invasive diagnostic modalities and gene expression profiles. Cancer 2004. © 2004 American Cancer Society.