Cigarette smoking (CS) and air pollution can both alter autonomic cardiac function, yet little has been reported on their combined effect on cardiovascular dysfunction. Therefore, we assessed the potential effect of smoking on the association between sulfur dioxide (SO(2)) and heart rate variability (HRV) in community residents. We found evidence that SO(2) induced short but dramatic decreases in HRV indices, the standard deviation of the NN interval (SDNN), low frequency (LF), and high frequency (HF), in smokers compared with non-smokers. We suggest that CS influences the decrease in HRV directly caused by SO(2), and the effect in susceptible groups may be more serious.