Prolonged exposure to estrogens correlates with an increased risk for breast cancer. One explanation is that estrogen metabolites cause mutations by reacting with DNA, leading to depurination. We describe an extraction procedure and a liquid chromatographic tandem mass spectrometric (LC/MS/MS) assay to detect estrone-metabolite-modified adenine (Ade) in 100-200 mg samples of human breast tissue. To ensure reliable analyses, we used a synthetic estrone-metabolite-modified, U-15N-labeled Ade as an internal standard (IS). Appropriate high-pressure liquid chromatography gives sharp (∼5 s at half-height) and identical retention times for the analyte and the IS. In breast tissue from women with and without cancer, we found a coeluting material with similar MS/MS fragmentation as the IS, providing high specificity in the identification of the modified Ade; the recovery was approximately 50%. For women with and without breast cancer, the levels of the modified Ade are in the range of 20-70 fmol/g of breast tissue from five women and not detectable in tissue from another woman. The sample size and detection limits are not yet sufficient to permit distinctions between cancer and noncancer patients.