There is increasing evidence to suggest that O6-alkyl guanine DNA-alkyltransferase (MGMT) activity provides protection against alkylating agent induced formation of GC→AT transition mutations in the K-ras oncogene of colorectal tumours. As this mutagenic event occurs during the growth of adenomas, both biomarkers of exposure (N7-methylguanine levels in DNA) and susceptibility (MGMT activity) were measured in biopsy samples obtained from normal and adenomatous tissue from 34 patients with large adenomas (>10 mm in size). There was no correlation between MGMT activity in the adenoma and in matched normal tissue. However, MGMT activity was significantly lower in adenoma tissue than in adjacent normal mucosa (5.18 versus 7.05 fmol/μg DNA, P = 0.01), particularly in men and those whose age was greater than the median. Upon stratification by K-ras mutational status, MGMT activity was lower in adenomas bearing a K-ras GC2[AT transition mutation (mean 4.21 fmol/μg DNA) than in adjacent normal tissue (mean 7.7 fmol/μg DNA; P <0.004). In contrast, there was no significant difference in MGMT activity in adenomas lacking a K-ras GC→AT transition mutation and adjacent normal mucosa. N7-methylguanine levels however did not vary with age, gender, K-ras mutational status or MGMT activity. These results are consistent with the acquisition of K-ras GC→AT transition mutations in adenomas with low MGMT activity as a result of unavoidable exposure to methylating agents. © Oxford University Press 2004; all rights reserved.