Cancer cells have been isolated from the circulating blood of cancer patients.1-3The viability of these cells and their subsequent role in metastases formation is still under question. Moore, et al.4and McDonald, et al.5have reported the growth of tumor cells from the blood in tissue culture. The very occurrence of blood-borne metastases indicates that some of these cells are viable and capable of growth. The prognostic importance of the identification of tumor cells in the circulating blood of cancer patients is still unknown. Engell6reported that 51% of patients surviving 5 years or longer after the surgical treatment of cancer had tumor cells identified in their blood prior to the treatment. Methods Of patients with various types of malignant diseases, 574 have been followed for 18 months or more after either curative surgery and adjuvant chemotherapy, palliative surgery, or anticancer chemotherapy. Peripheral blood samples