AIMS: Epidemiological studies show that peak exposure to air pollution is associated with increased morbidity and mortality from cardiovascular events. Panel and controlled exposure studies show that particulate matter (PM) may influence the parasympathetic regulation of the heart. The aim of this study was to concurrently measure individual exposure to PM of various sizes, heart rate variability (HRV), and electrical instability in patients with myocardial infarction. METHODS AND RESULTS: Personal exposures to PM(10), PM(2.5), and PM(0.25) was measured over 24 h in 39 patients (36 males, 3 females; mean age 60.3 years) with prior myocardial infarction (>6 months). Simultaneously, a 24 h ECG was recorded and then analysed for HRV and ventricular arrhythmias. Breath condensate and blood samples also were collected at the end of monitoring to measure several indexes of inflammation. Negative correlation was found between HRV and exposure to PM(0.25) in a group of patients not taking beta-blockers. More severe ventricular arrhythmias were observed at the highest concentrations of PM(10) and PM(2.5). Indexes of inflammation in either breath condensate or blood did not correlate with PM exposures. CONCLUSION: Our study shows that exposure to ultrafine particles is associated with autonomic dysregulation in selected patients with myocardial infarction. More severe arrhythmias occur at the highest exposures to larger particles. Nevertheless, the underlying mechanisms remain hypothetical because inflammation may be evoked by PM or be related to the disease itself.