This study was planned to test the hypothesis that patients with congestive heart failure are more susceptible to the harmful effects of ambient air pollution than the general population. We used both general additive Poisson regression and the case-crossover approach to test the hypothesis. The effect of air pollution on daily mortality of patients with congestive heart failure among residents of Seoul, South Korea, during the period 1994--1998 was compared with that of the general population in the same area and the same period. The odds ratios and 95% confidence intervals (95% CIs) estimated from general additive models in the general population for an interquartile range increase of particulate matter less than 10 microm in diameter (42.1 microg/m(3)), carbon monoxide (0.59 ppm), nitrogen dioxide (14.6 ppb), sulfur dioxide (9.9 ppb), and ozone (20.5 ppb) were 1.014 (95% CI = 1.006- 1.022), 1.022 (95% CI = 1.017- 1.029), 1.021 (95% CI = 1.014- 1.029), 1.020 (95% CI = 1.012--1.028), and 1.010 (95% CI = 1.002--1.017), respectively. The estimated effects appeared larger among the congestive heart failure patients than among the general population (2.5 approximately 4.1 times higher depending on the pollutants). The results from the case-crossover analysis were similar. The finding of a stronger association in the patients with congestive heart failure reinforces the evidence that a harmful effect of air pollution is mediated by cardiovascular mechanisms.