Copper smelters are large, usually isolated, sources of air pollution. Arizona has several such plants on the periphery of small communities. The smelters emit predominantly sulfur oxides and particulates, and the residents of these communities intermittently are exposed to high concentrations of smelter smoke but little other pollution. This study compared the respiratory health of Anglo-American school children who lived in one smelter community with children living in another small community in Arizona that was free of smelter air pollution.