DNA aneuploidy is common in colon carcinoma, colonic polyps, and ulcerative colitis. We found an interesting subset of patients with colon cancer. These individuals (14 of 230 cases, 6%) had hypodiploid aneuploidy in their morphologically normal-appearing colonic tissue. The aneuploid fractions were small, making up between 10 to 15% of the total events, and the ploidy pattern was not related to the ploidy pattern of the tumor. The clinical findings of the 14 patients were compared with those of patients who had diploid normal colons by age, location of the tumor, Dukes stage, and percent aneuploidy in the main tumor; the two groups were comparable. Both the normal colon and carcinomas of the 14 cases were studied by a newly developed "enriched" polymerase chain reaction for K-ras mutations. No K-ras mutations were found in the normal tissues, but mutations were found in the tumors of four cases. Cells from some colon cancers have a high degree of genetic instability, as shown by numerous mutations throughout the genome. Analysis of eight matched colon cancers and aneuploid, morphologically normal-appearing colons for genetic alterations, as measured by shifts in the electrophoretic mobility of microsatellite repeat fragments, showed that only one of eight colon cancer samples had microsatellite instability, which is the expected frequency. Taken together, the data suggest that individuals with colon cancer may have morphologically normal colonic tissue, which is genetically abnormal, and this abnormality may precede the development of mutations in K-ras.(ABSTRACT TRUNCATED AT 250 WORDS)