Abstract: A method and kit for detecting breast cancer cells is disclosed, which is not only as convenient and noninvasive as conventional cytological examination, but also is more accurate and objective than cytological examination. The invention establishes a method for detecting breast cancer cells, wherein the method judges cells examined to be breast cancer cells when aneusomy is detected on 1) either chromosome 1 or chromosome 11 or both, 2) either chromosome 1 or chromosome 17 or both or 3) at least any one of chromosomes 1, 11 and 17 of the cell sample.