The autopsies of 302 patients who had metastases from cancers of unknown primary site were studied. In two-thirds of the cases the metastases were located in the lymph nodes, the lungs and the bones. The primary tumour was found in 82 patients when still alive (27 percent) and in 173 patients (57 percent) at autopsy. It could not be identified in 16 percent of the cases. The primary tumours most often discovered were in the pancreas (26.5 percent), the lungs (17.2 percent), the kidneys (4.6 percent) and the colorectal bowel (3.6 percent). On the whole, the paraclinical examinations performed for diagnostic purposes had been disappointing. The survival rate was the same whether or not the primary tumour had been identified. The number of inaugural metastases seems to be a major prognostic factor. These highly progressive tumours must represent a very distinct clinical and biological neoplastic entity. A simple approach and a unicist practical management are suggested.