While 50% of colorectal tumors were found to harbor K-ms codon 12 mutation, only 18% of the respective patients contain this mutation in the normal appearing tissue when enriched PCR, a sensitive method that enables detection of one mutant allele in 10(4) normal alleles, is used. To determine whether the lower percentage could be attributed to the low incidence of this mutation or to the method of sampling, we have analyzed multiple normal appearing samples obtained from the same patient. Of 90 non-neoplastic mucosal samples collected from 20 patients with colorectal cancer, K-ras codon 12 mutation was identified in 6 samples taken from 5 patients. These results indicate that only one or two of the multiple samples contain mutant ras alleles. The presence of mutant ras alleles in the normal appearing tissues did not always correlate with that in the tumor site with respect to its presence and/or the type of base pair alterations, indicating independent or late events. While pointing to the importance of proper sampling method, the ability to detect ms mutation in normal mucosal tissues suggests it may serve as a useful biomarker of internal/external exposure which precede colorectal cancer development.