Carcinocythemia, the presence of circulating cancer cells in peripheral blood, is a rare complication of solid neoplasms. When the number of such cells is very high, they can be detected during routine laboratory tests. They are associated with a dismal prognosis. Carcinocythemia occurred in a patient with disseminated breast cancer. Eighteen cases were identified from a review of the literature. The most common neoplasms associated with circulating cancer cells in peripheral blood were breast adenocarcinoma, small cell lung carcinoma and rhabdomyosarcoma. All the patients had stage IV disease at the time of diagnosis, and all had involvement of the reticuloendothelial system. Patients survived for an average of a few days or weeks. Circulating cancer cells in peripheral blood are an unusual manifestation of disseminated neoplasms that occurs as a terminal event.