We studied 302 consecutive autopsied patients who presented with carcinoma of unknown primary origin. The most frequent metastatic sites were the nodes, lung, and bone. The primary site was identified while patients were alive in 27% and at autopsy in 57%; the site remained unidentified in 16%. The pancreas (26.5%), lung (17.2%), kidney (4.6%), and colorectum (3.6%) were the most frequent primary sites, but the reliability of diagnostic tests used in the search for this site was disappointing. Survival was identical in patients whose primary site was discovered while alive, at autopsy, or remained unknown. The number of metastases at presentation was the major prognostic factor. Analysis of autopsy data demonstrated that patients with carcinoma of unknown primary origin pursue a different course than expected when the primary site is the first manifestation of the disease. On the basis of these results and the results of other modern series, we suggest an approach consisting of a limited initial workup but with greater emphasis on modern histochemistry studies and immunohistopathologic and other kinetic and morphologic parameters to understand the patient tumor characteristics better and base the clinical management on an individual basis.