Patients after coronary artery bypass grafting (CABG) have impaired heart rate variability (HRV) parameters. The clinical significance of impaired HRV and the influence of the cardiac rehabilitation on the HRV in this group of patients is not clear. We studied 103 patients (average age 52.2 +/- 7.8) after CABG, divided into group A (72 patients) who underwent cardiac rehabilitation (starting 15 days after the CABG) and group B (31 patients) who did not have rehabilitation. In all patients, a 24 hr ECG (including time and frequency domain HRV) was obtained before the onset of rehabilitation, after completion of the rehabilitation and after 6 months. Before the rehabilitation there were no differences in time-domain HRV between the groups, however frequency-domain HRV values were significantly higher in group A. In group A, the time-domain HRV parameters improved after 21 days of rehabilitation, whereas the frequency domain parameter components remained unchanged. At 6 months, improvement in time and frequency domain HRV parameters was noted, but the parameters were significantly higher in group A. In patients that underwent coronary artery bypass grafting, the cardiac rehabilitation time domain HRV showed improvement in both the short term (21 days) and long run (6 months). However, the frequency domain parameter was only increased over the long run. Cardiac rehabilitation speeds up the improvement of the time domain HRV and frequency domain parameters over long term observation (6 months).