The heart period duration was measured 201 times for 1,024 heart beats (beat-to-beat intervals) in 96 patients with severe cerebral damage. The standard deviation and the coefficient of the variability was calculated, a Fast-Fourier-Analysis made and the power spectrum represented. The prognostic value of heart rate variability (HRV) and of the power spectrum and their usability for brain death diagnosis were examined. It was shown that: 1st: between patients at the time of a clear clinical deterioration and brain dead patients significant differences existed both in the levels of deviation and in the absolute values of the power spectrum, 2nd: the examined spectral fields were not influenced to the same intensity, 3rd: the decreases of the values of variability and the absolute value of all spectral fields, but especially of the percentage part of the field B, presented a prognostically bad sign and 4th: controlled ventilation also in patients with cerebral areflexia considerably influenced the determined values depending on the breathing volume.