Recent time-series epidemiological studies have reported significant associations between daily air pollution and mortality. These studies typically report a short-term excess increase in deaths as the fractional increase of total (nonaccidental) deaths per unit of air pollutant. The relative risk (RR) calculated for the total population in these studies, however, may underestimate the risk for the most sensitive subpopulation(s) at risk. In this study, race, gender, and cause-specific counts of daily mortality in Cook County, Illinois (which encompasses the city of Chicago) during 1985-1990 were analyzed to determine if there was any heterogeneity in air pollution/weather/mortality associations across these various population subcategories. Seasonal cross-correlations between mortality and environmental variables first were examined to identify appropriate lag structures. Of the pollution variables considered-particulate matter less than 10 microns (PM10), ozone, carbon monoxide, sulfur dioxide, and visual range-derived extinction coefficient-both PM10 and ozone showed significant associations with same-day and next-day mortality. The Poisson regression models employed included seasonal cycles (sine/cosine series), square and linear terms of lagged temperature, trend line, day-of-week dummy variables, and the average of the same day's and previous day's PM10 or ozone. The RR for total nonaccidental mortality per 100 micrograms/m3 increase in PM10 was 1.05 (95% CI: 1.03-1.08). The respiratory (RR = 1.14; 95% CI: 1.04-1.25) and cancer (RR = 1.12; 95% CI: 1.06-1.18) categories showed higher estimates than the circulatory category (RR = 1.03; 95% CI: 0.98-1.07), while the residual of the total from these three categories showed no association with PM10 (RR = 1.01; 95% CI: 0.95-1.08). Among the race- and gender-specific categories, black (African-American) females showed the higher RRs for the total (RR = 1.11; 95% CI: 1.03-1.21), respiratory (RR = 1.31; 95% CI: 0.98-1.75), and cancer (RR = 1.25; 95% CI: 1.07-1.46) mortality categories. Neither ozone nor hot temperature showed such cause-specificity in mortality associations. Cold temperature lagged by two days was a significant predictor of circulatory and respiratory mortality. This study suggests the importance of race- and gender-specific analysis. The greater mortality risk to Chicago's black women from exposure to urban air pollution indicated by this finding should be tested in other metropolitan areas.