Anomalies of autonomic control of the coronary circulation may play a role in the development of syndrome X (angina pectoris, ischemic-appearing results on exercise test, and normal coronary arteriograms). Twenty-six patients with syndrome X and 20 healthy sex- and age-matched control subjects were studied by means of analysis of heart rate variability during 24-hour Holter monitoring. Spectral and nonspectral parameters of heart rate variability were investigated. Mean heart rate was similar in patients with syndrome X and in control subjects. Patients with syndrome X had significantly lower standard deviation of all normal RR intervals, a lower percentage of adjacent normal RR intervals > 50 ms in difference (126.4 +/- 22 vs 149 +/- 43 ms, p < 0.05; 6.3 +/- 4 vs 11.2 +/- 7%, p < 0.05; respectively), and a trend toward lower values of time-domain parameters. Lower values of total power and low frequency were also observed in patients with syndrome X (1273 +/- 693 vs 1790 +/- 989 ms2, p < 0.05; 406 +/- 176 vs 729 +/- 455 ms2, p < 0.01, respectively). An inverse correlation between heart rate and measures of heart rate variability was found in syndrome X but not in control subjects. High- and low-frequency power showed a similar circadian pattern in syndrome X patients and control subjects. Patients and control subjects were then allocated into 2 groups according to the median RR duration: syndrome X1 and control 1 with high mean heart rate, and syndrome X2 and control 2 with low mean heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)