Sixty-five tumors of invasive ductal carcinoma of the breast were examined for p53 alteration by the reverse transcription-polymerase chain reaction-single strand conformation polymorphism (RT-PCR-SSCP) method and sequencing analysis. In total, 16 samples (24.6%) showed p53 gene alteration. Sixteen of these alterations were evaluated by sequencing analysis, and 15 showed missense point mutations while one showed a 9-base pair deletion. In the 15 point mutations, G:C to A:T transitions constituted the majority (53%), and five tumors (33%) had a transition at the CpG site, which are mutational patterns not commonly found in breast tumors from Europe and America. On the other hand, there were no G:C to T:A transversions in our cases, which were frequently observed transversions in Europe and America. These p53 mutation patterns in breast cancer in Japan are not similar to those in Europe and America reported by Hollstein et al. and Coles et al.. These findings suggest that there are some differences between mechanisms of breast cancer in Japan and in Europe and America.