Cities in northern Chile had arsenic concentrations of 860 mg/liter in drinking water in the period 1958–1970. Concentrations have since been reduced to 40 mg/liter. We investigated the relation between lung cancer and arsenic in drinking water in northern Chile in a case-control study involving patients diagnosed with lung cancer between 1994 and 1996 and frequency-matched hospital controls. The study identified 152 lung cancer cases and 419 controls. Participants were interviewed regarding drinking water sources, cigarette smoking, and other variables. Logistic regression analysis revealed a clear trend in lung cancer odds ratios and 95% confidence intervals (CIs) with increasing concentration of arsenic in drinking water, as follows: 1, 1.6 (95% CI 5 0.5–5.3), 3.9 (95% CI 5 1.2–12.3), 5.2 (95% CI 5 2.3–11.7), and 8.9 (95% CI 5 4.0–19.6), for arsenic concentrations ranging from less than 10 mg/liter to a 65-year average concentration of 200– 400 mg/liter. There was evidence of synergy between cigarette smoking and ingestion of arsenic in drinking water; the odds ratio for lung cancer was 32.0 (95% CI 5 7.2–198.0) among smokers exposed to more than 200 mg/liter of arsenic in drinking water (lifetime average) compared with nonsmokers exposed to less than 50 mg/liter. This study provides strong evidence that ingestion of inorganic arsenic is associated with human lung cancer.