Carcinocythemia (carcinoma cell leukemia) has been previously described as a rare, late and dramatic event occurring in widespread tumoral disease. We report a case of carcinocythemia occurring in a patient with a particularly indolent breast cancer. When a large amount of circulating tumor cells (CTC) appeared in the blood smears, neither visceral macrometastases nor massive bone marrow infiltration could be detected. We isolated CTC to perform cell cycle analysis and to study the expression of adhesion molecules. A fraction of the CTC was proliferating in the bloodstream but we detected no relevant anomaly of adhesion properties in the CTC. A post-mortem biopsy disclosed an exclusive sinusoidal infiltration of the liver. We propose that in this case, carcinocythemia resulted from the release of CTC from the visceral microcirculation where the proliferation occurred.