Inorganic arsenic (In-As) is known to be a human carcinogen, causing lung cancer by inhalation and skin cancer by ingestion. Ecologic studies in Taiwan have found a dose-response relation between ingestion of In-As from drinking water and bladder cancer, but questions have been raised concerning the validity and generalizability of the findings. Several areas of Argentina have had high exposures to arsenic from naturally contaminated drinking water, particularly the eastern region of the province of Córdoba. In this study, we investigated bladder cancer mortality for the years 1986-1991 in Córdoba's 26 counties, using rates for all of Argentina as the standard for comparison. Bladder cancer standardized mortality ratios (SMRs) were consistently higher in counties with documented arsenic exposure. We grouped counties into low-, medium-, and high-exposure categories; the corresponding SMRs [with 95% confidence intervals (CI)] were 0.80 (95% CI = 0.66-0.96), 1.42 (95% CI = 1.14-1.74), and 2.14 (95% CI = 1.78-2.53) for men, and 1.21 (95% CI = 0.85-1.64), 1.58 (95% CI = 1.01-2.35), and 1.82 (95% CI = 1.19-2.64) for women. The clear trends found in a population with different genetic composition and a high-protein diet support the findings in Taiwan.