Twenty-six patients with untreated mild to moderate Parkinson's disease (PD) lasting 5 years or less, were investigated in the sleep laboratory. Sleep architecture and respiration during sleep did not significantly differ from age-matched controls. PD patients showed defective cardiac autonomic control during sleep, mainly parasympathetic but also sympathetic in nature, although they had normal results in conventional autonomic tests during wakefulness. Therefore, autonomic cardiac involvement seems to be an early sign of PD.