The relationship between daily mortality of elderly (65+ y) persons and air pollution in the metropolitan area of Sao Paulo, Brazil, for the period May 1990 to April 1991 was evaluated by time series regression, controlling for season, weather, and other factors. Mortality was associated with respirable particles (PM10), nitrogen oxides (NOx), sulfur dioxide (SO2), and carbon monoxide (CO). The association with PM10 was most statistically significant, robust, and independent of other air pollutants. An increase in PM10 equal to 100 micrograms/m3 was associated with an increase in overall mortality equal to approximately 13%. This association was consistent across various model specifications and estimation techniques. The dose-response relationship between mortality and respirable particulate pollution was almost linear, with no evidence of a "safe" threshold level. The results were similar to those observed in London and several U.S. cities. The results were also supportive of recent animal studies that have observed adverse health outcomes in experimental animals exposed to air pollution in Sao Paulo.