Automatic search for cytokeratin/mucin-1 double immunofluorescence was performed to detect and characterize circulating epithelial tumor cells in patients with advanced breast cancer. The peripheral blood samples in 8 of 19 patients (42.1%) presented with cytokeratin-positive and epithelial-type mucin-positive (CK(+)/MUC1(+)) tumor cells. Detailed microscopic analysis, however, suggested that the majority of the double immunopositive cells was apoptotic according to an "inclusion type" cytokeratin staining pattern and nuclear condensation. Furthermore, apoptosis-related DNA strand breaks could be demonstrated by applying the TdT-uridine nick end labeling assay in these cells. In 3 of 8 positive samples all of the CK(+)/MUC1(+) cells displayed apoptotic features. We conclude that apoptotic cells significantly contribute to the circulating tumor cell fraction in breast cancer patients. As the predictive value of such cells for the outcome of the disease is unclear, they should be considered separately when analyzing tumor cell dissemination.