There have been no reports of studies on nocturnal sleep or quantitative studies of sleep spindle density in Huntington's disease, whereas sleep in Parkinson's disease has been extensively reported. We therefore examined nocturnal sleep changes in patients with Parkinson's and Huntington's disease and compared them with a control group. The previously reported decrease in sleep spindle density was observed in patients with Parkinson's disease. The patients with Huntington's disease, however, had a significant increase in sleep spindle density, the nocturnal sleep pattern being normal in other respects when compared with the control group. This observation may provide a further diagnostic criterion in Huntington's disease.