A quantitative, noninvasive method of assessing autonomic control, based on the spectral analysis of beat-to-beat fluctuations in heart rate (HR), was applied to patients with chronic renal failure (RF). Since the power spectrum of HR fluctuations measures the dynamic nervous control of HR, it can be used to quantitate a normal control system as opposed to a disturbed or depressed system. Indeed, in RF patients, a strong reduction in the HR power spectrum was observed in all frequency ranges, both sympathetically and parasympathetically mediated. A similar depression in autonomic control was demonstrated in patients on hemodialysis or peritoneal dialysis. RF patients not yet undergoing dialysis show a lesser degree of depression. Spectral analysis of HR fluctuations in RF patients makes it possible to quantitate autonomic dysfunction and to reliably measure its development as a function of time, and requires only a 10-min standard electrocardiogram recording.