Objective: We sought to assess whether the metabolism of arsenic impacts a person's susceptibility to bladder cancer. Methods: Urinary methylation products were measured in subjects from Argentina (114 cases and 114 controls) and the United States (23 cases and 49 controls). Results: In Argentina, the adjusted odds ratio (OR) for subjects with a high proportion of ingested arsenic excreted as monomethylarsonate (%MMA) was 2.17 (95% confidence interval [CI] = 1.02-4.63) in smokers and 0.48 (95% CI = 0.17-1.33) in nonsmokers. In the United States, the adjusted ORs for high %MMA in subjects with arsenic intakes less than and greater than 100 μg/d were 1.20 (95% CI = 0.27-5.38) and 2.70 (95% CI = 0.39-18.6). Conclusions: Overall, these results are consistent with data from Taiwan suggesting that some individuals who excrete a higher proportion of ingested arsenic as MMA are more susceptible to arsenic-related cancer.Fil: Steinmaus, Craig. Arsenic Health Effects Research Program, School of Public Health, University of California, Berkeley, CA, United StatesFil: Bates, Michael N. Arsenic Health Effects Research Program, School of Public Health, University of California, Berkeley, CA, United StatesFil: Yuan, Yan. Arsenic Health Effects Research Program, School of Public Health, University of California, Berkeley, CA, United StatesFil: Kalman, Dave. School of Public Health and Community Medicine, University of Washington, Seattle, WA, United StatesFil: Atallah, Raja. School of Public Health and Community Medicine, University of Washington, Seattle, WA, United StatesFil: Rey, Omar A. Universidad Católica de Córdoba. Facultad de Ciencias de la Salud; ArgentinaFil: Biggs, Mary Lou. Arsenic Health Effects Research Program, School of Public Health, University of California, Berkeley, CA, United StatesFil: Hopenhayn, Claudia. College of Public Health, Markey Cancer Control Program, University of Kentucky, Lexington, KY, United StatesFil: Moore, Lee E. Arsenic Health Effects Research Program, School of Public Health, University of California, Berkeley, CA, United StatesFil: Hoang, Bruce K. Division of Occupational and Environmental Medicine, University of California, San Francisco, CA, United StatesFil: Smith, Allan H. Arsenic Health Effects Research Program, School of Public Health, University of California, Berkeley, CA, United States