The authors used a case-crossover approach to evaluate the association between ambient air pollution and the rate of hospitalization for congestive heart failure among Medicare recipients (aged > or =65 years) residing in Allegheny County (Pittsburgh area), Pennsylvania, during 1987-1999. They also explored effect modification by age, gender, and specific secondary diagnoses. During follow-up, 55,019 patients were admitted with a primary diagnosis of congestive heart failure. In single-pollutant models, particulate matter with an aerodynamic diameter of <10 microm (PM(10)), carbon monoxide, nitrogen dioxide, and sulfur dioxide-but not ozone-were positively and significantly associated with the rate of admission on the same day. The strongest associations were observed with carbon monoxide, nitrogen dioxide, and PM(10). The associations with carbon monoxide and nitrogen dioxide were the most robust in two-pollutant models, remaining statistically significant even after adjusting for other pollutants. Patients with a recent myocardial infarction were at greater risk of particulate-related admission; otherwise, there was no significant effect modification by age, gender, or other secondary diagnoses. These results suggest that short-term elevations in air pollution from traffic-related sources may trigger acute cardiac decompensation in heart failure patients and that those with certain comorbid conditions may be more susceptible to these effects.