Recently, two different mechanisms of genetic instability have been demonstrated in the carcinogenesis of colorectal cancer. Microsatellite instability is an important genetic event for carcinogenesis in hereditary non-polyposis colorectal cancer, proximal colon cancer, and multiple colorectal carcinoma. To examine the association among chromosomal instability and multiple primary malignancies (MPM) in colorectal cancer, fluorescence in situ hybridization using a chromosome 17-specific probe, p53 cosmid probe, and/or an alpha satellite DNA probe was performed in 184 patients with colorectal cancer. The proportion of aneusomy 17 in MPM was significantly higher than that of single cancers (SC) (46.1+/-8.0% and 39.0+/-10.3%, respectively; p<0.01). Multiple numerical aberrations of chromosome 17 in MPM occurred more often than those of SC (64.3% and 22.9%, respectively; p<0.01). The mean frequency of p53 deletion was also higher in MPM (70.4+/-16.7%) compared with SC (53.4+/-18.1%, p<0.05). The frequency of chromosome 17 translocation was significantly greater in tumors with MPM (4/6; 67%) than in SC (3/23; 13%, p<0.05). The frequency of p53 locus translocation was also significantly greater in tumors with MPM (4/6; 67%) than in SC (0/23; 0%, p<0.01). These results suggested that numerical and structural aberrations of chromosome 17 and the p53 locus are important genetic events associated with carcinogenesis in non-familial colorectal cancer with MPM.