Observation of a unique population of cells on a Wright-stained blood smear of a patient with metastatic breast carcinoma prompted a study to determine their origin. The primary carcinoma contained a marker, the presence of "signet cells." These were demonstrated in direct peripheral smears and buffy coat preparation of peripheral blood and confirmed histochemically by showing positive periodic acid-Schiff, alpha-napthol and beta-glucuronidase reactions. "Carcinocythemia" is suggested as a name for this unusual process observed over a six month period. Studies of the patient's immunocompetence, of circulating cell surface immunoglobulins and karyotype analysis were made. Postmortem examination revealed retroperitoneal fibrosis, splenic atrophy and extensive metastatic carcinoma but no evidence of leukemia. The cells will be contrasted to those seen in a second patient who appeared to have acute myelocytic leukemia complicating extensive cancer involving the bone marrow. The observations suggest that a leukemia-like blood picture due to circulating cancer cells may occur during the course of metastatic breast carcinoma.