The amplitude of the H-reflex has been known to have considerable variability even if the intensity of the stimulation is held constant. However, previous studies largely ignored the time-dependent profile of this variability. Recent mathematical analyses have shown that some seemingly irregular biological signals have fractal properties. A fractal time series is characterized by the property of self-similarity (self-affinity), and has long-range time correlation. The aim of this study was to investigate the question of whether the fluctuation of H-reflex was fractal with strong time-correlation. Soleus H-reflexes were evoked in five healthy subjects at two levels of stimulation intensity [1.2 MT (motor threshold) and 0.9 MT] every 1 s and 1050 successive amplitudes of H-wave and M-wave were recorded twice. The sequences of the H-wave and the M-wave amplitudes were analyzed by "coarse graining spectral analysis" to calculate the percentage of random fractal components in the sequences (%Fractal) and the spectral exponent beta. The %Fractal values of both sequences were above 90% [H-wave: 93.3 +/- 2.3% (1.2 MT), 91.6 +/- 3.1% (0.9 MT); M-wave: 94.3 +/- 3.3%; mean +/- SD]. Nonflat power spectra of the fractal components were observed for the H-wave sequences regardless of the stimulation intensity [beta = 0.75 +/- 0.26 (1.2 MT), 0.80 +/- 0.39 (0.9 MT)], indicating that the sequences were strongly time correlated. On the other hand, the M-wave sequences had a flatter spectrum (beta = 0.26 +/- 0.14) which was close to uncorrelated white noise. These results indicated that: (1) the fractal correlation found in the H-wave sequences was caused neither by the conduction through nerve fibers nor by the transmission at the neuromuscular junction, because the M-wave sequence had a significantly weaker time correlation, and (2) antidromic impulses in a motor nerve induced by the stimulation made a minor contribution to the generation of fractal correlation in the H-wave sequences, because it was preserved when the stimulation intensity was below MT. It was suggested that the fractal correlation in human H-reflex was generated at the synaptic connections to alpha-motoneurons in the spinal cord.