['To determine whether exposure to methylating agents was a risk factor for treatment failure in women undergoing colposcopic examination.', 'Nine hundred fifty-eight women attending for colposcopic examination after abnormal cervical smear test results were recruited into the study cohort. Information on demographic factors, smoking and other risk factors was obtained and a pre-treatment biopsy was taken and stored at -70 degrees C. After follow-up, cases who had treatment failure of cervical intraepithelial neoplasia (CIN) within 2 years following treatment were identified (n = 77) and matched to women with no treatment failure of CIN in this time period (controls, n = 154). DNA was extracted from the pre-treatment biopsies and levels of N7-methyl-deoxyguanosine (N7-MedG), a marker of exposure to methylating agents, were quantified as the ring-opened form of the base damage by a validated immunoslotblot assay.', 'Sufficient DNA for N7-MedG analysis was extracted from 61 subjects corresponding to 20 matched case control pairs. N7-MedG was detected in cervical DNA with levels ranging from non-detected (<0.1 micromol/mol dG) to 4.83 micromol/mol dG. N7-MedG levels were significantly higher in cases (geometric mean 0.99 micromol/mol dG) than controls (0.33 micromol/mol dG; P = 0.01). There were no associations between N7-MedG levels and HPV or smoking status. Log N7-MedG content, after adjustment for HPV status at time of treatment, was found to be significantly associated with increased risk of treatment failure (OR 5.74, 95% CI 1.05-31.23).'] Are levels of the DNA adduct , N7-methyldeoxyguanosine , associated with increased risk of failure of treatment of cervical intraepithelial neoplasia?