The expression and serum level of NCC-ST-439, a tumor marker, has been investigated in 20 primary and 28 recurrent breast cancer patients, and the positive rate was found to amount to 21.1% in primary cases and 42.9% in recurrent cases, respectively, thereby indicating a positive correlation between the expression of NCC-ST-439 and the progression of the diseases. The accordance rate between NCC-ST-439 and CEA was 64.6%; in contrast, the rate between NCC-ST-439 and CA 15-3 was 82.1%, suggesting a difference in specificity against tumor between NCC-ST-439 and CEA. The change in the serum level of NCC-ST-439 in recurrent cases clearly correlated with the effectiveness of the therapy. These findings suggest that measurement of the NCC-ST-439 level, especially in combination with the CEA level, may be useful for the early detection and the monitoring of relapses in breast cancer patients.