Epidemiological studies have shown that air pollution is associated with increased cardiovascular mortality. Although the pathophysiological mechanisms behind this association have remained largely unknown, it has been suggested that changes in cardiac autonomic function may play a role. In this study, we investigated the association between acute carbon monoxide (CO) exposure and cardiac autonomic function as measured by heart rate variability (HRV) in subjects with stable coronary artery disease. Twenty-four hour ambulatory electrocardiographic recording with simultaneous continuous personal CO concentration monitoring was performed in six male patients with angiographically verified coronary artery disease three times with 1-week intervals. Time domain measures of HRV were calculated for 5-min segments before and during the CO exposure periods. For further analysis CO exposures were divided into low (</=2.7 p.p.m.) and high (>2.7 p.p.m.) CO exposure periods. The mean of maximum CO levels during 61 CO exposure periods was 4.6 p.p.m. (SD 5.0 p.p.m.). High CO exposure was associated with an increase in the square root of the mean of the sum of the squares of differences between adjacent RR intervals (r-MSSD) (P = 0.034). Heart rate remained unchanged during the CO exposure. In conclusion, acute CO exposure which represented most likely exposure derived from traffic seems to modify cardiac autonomic control in patients with stable coronary artery disease.