In our previous study, amplification of c‐erbB‐2 and c‐mycproto‐oncogenes in DNA of human breast cancer occurred in 16% and 4% of cases, respectively, and increased copy number of these genes is suggested to be associated with aggressive primary tumors. We examined change in the copy number ofc‐erbB 2 andc‐mycproto‐oncogenes between primary and multiple metastatic tumors in 10 patients with breast cancer, who underwent breast surgery and were later autopsied, by using DNAs isolated from formalin‐fixed paraffin‐embedded tissues and the dot blot‐hybridization method. In primary tumors, amplification of c‐erbB‐2 and c‐myc was detected in three and two cases, whereas at the stage with systemic metastasis, it was detected in four and three cases, respectively. In all four cases with amplifiedc‐erbB‐2gene and in one of the three with amplified c‐myc gene, the copy number was clearly increased in the metastatic tumors in comparison with the primary. Microscopically, more than five mitotic figures per high power field were detected in metastatic tumors of five cases including three with amplified c‐erbB‐2, but in only two primary. These results suggested that the aggressive nature of breast cancer is frequently enhanced in accordance with cancer metastas