We investigated urinary mutagenicity and colorectal adenoma risk in a clinic-based, case-control study of currently nonsmoking cases (n = 143) and controls (n = 156). Urinary organics were extracted by C18/methanol from 12-h overnight urine samples, and mutagenicity was determined in Salmonella YG1024 +S9 (Ames test). Adenoma risk was 2.4-fold higher in subjects in the highest versus the lowest quintile of urinary mutagenicity (95% confidence interval = 1.1-5.1). Combining urinary mutagenicity with intake of meat-derived mutagenicity (from our earlier analysis) resulted in a 5.6-fold increase in adenoma risk (95% confidence interval = 2.2-13.9, comparing the highest with the lowest quintile). In our study population, diet may have contributed to mutagenic exposure, which was positively associated with colorectal adenoma risk.