From 1958 to 1970 the 270,000 inhabitants of Antofagasta City received drinking water containing 850 ug/L of arsenic until health effects attributed to arsenic were identified and prompted its control. Beginning in 1970 water treatment plants lowered arsenic concentrations to 110 ug/L, 70 ug/L, 40 ug/L and 10 ug/L in the 1970’s, 80’s, 90’s and 2003 respectively. Arsenic-associated short-term effects first occurred in children 11–15 years old consisted of skin arsenicism (50%) and respiratory symptoms (8%), systemic vascular disease, and infant mortality. Rates of these effects returned to expected levels in the 1970’s. Twenty to 40 years after exposure, the rates of skin, bladder, lung, kidney and liver cancers increased. An exception was breast cancer mortality which decreased during the high exposure period, returning to normal by the end of the 1970’s. Long term effects of arsenic exhibit a dose-response effect and increase by younger age of exposure, and with exposure to tobacco smoke, occupational dust and obesity. To date, neurological or developmental effects have not been studied in this population.