1. Insomnia is an even more frequent complaint in elderly patients with Parkinson's disease than might be expected from the effect of age alone on sleep. 2. A double-blind, placebo-controlled trial in eleven patients with Parkinson's disease of mean (s.d.) age 80(5) years, showed that nocturnal dosing with levodopa produced a clinically significant improvement in sleep both as assessed subjectively and by measurement of number of spontaneous moves in bed. 3. Despite the long interval between tablet administration and morning assessment, walking time was faster on mornings following active treatment.