Heart rate variability (HRV) is a measure of the physiological variation of R-R intervals, reflecting the sympathovagal balance. In both overt and subclinical hyperthyroidism, a relative increase in sympathetic activity has been demonstrated, mainly due to a decrease in vagal activity. The modifications of HRV during orthostatism in normal subjects resemble those seen in hyperthyroidism. We have studied the response of 19 patients with overt hyperthyroidism and 12 with subclinical hyperthyroidism during orthostatism using HRV and compared the results to those of 32 healthy controls. In the three groups, the R-R intervals decreased in the same proportion after orthostatism. The low frequency power (LF)/[LF + high frequency power (HF)] ratio, which reflects the sympathetic tone, also increased in the same proportion in the three groups. However, the mechanisms of the modulation of the sympathovagal balance during orthostatism were different among the three groups. In controls, the relative increase of sympathetic tone after orthostatism was due principally to a decrease in vagal tone (reflected by decreased power in the HF band), while in overt hyperthyroidism, where the power in the HF band was already minimal in the lying position, there was a clear increase in the LF band power during orthostatism. The results were intermediate in the subclinical hyperthyroidism group, reflecting a continuum of effects of the thyroid hormone excess on the autonomic nervous system. Our study shows that despite an apparent normal cardiovascular adaptation to orthostatism in hyperthyroidism, the modulation of the autonomic nervous system is profoundly modified.