The incidence of non-familial multiple primary cancer in colorectal cancer patients has increased in recent years in Japan. To clarify the characteristic genetic aberrations in such multiple cancers, we examined structural chromosomal aberrations by fluorescence in situ hybridization, using chromosome 17-specific and p53 cosmid DNA probes. We established short-term cultures of 78 surgical specimens and were able to obtain observable metaphase spreads in 23 single colorectal cancer specimens and in 6 colorectal cancer specimens from patients with double primary cancers. The frequency of chromosome 17 and/or p53 locus translocation was significantly greater in tumors with double cancer than in single colorectal cancers (P < 0.05 and P < 0.01, respectively). These aberrations in double cancers frequently appeared even at an early Dukes' stage (A and B) of colorectal carcinoma. Our results suggest that translocation of chromosome 17 and the p53 locus may be specific genetic events probably associated with carcinogenesis of multiple primary cancers in colorectal cancer.