Sleep disruption is a common complaint in levodopa-treated parkinsonian patients. A survey of 100 parkinsonian patients revealed prominent sleep complaints in 74%. Sleep complaints were unrelated to patient age and the duration of disease but increased in prevalence with longer periods of levodopa therapy. Sleep abnormalities tended to increase in severity with continued treatment and insomnia tended to be followed by daytime somnolence, altered dream events, and episodic nocturnal vocalization and myoclonus. While dyskinetic side effects and on-off syndrome were encountered in patients with and without sleep complaints, 98% of patients experiencing psychiatric side effects also reported sleep disruption. It is suggested that sleep-related symptoms constitute an early stage of levodopa-induced dopaminergic psychiatric toxicity in the parkinsonian population. Clinical and experimental observations suggest that serotonergic mechanisms are important in this symptom complex.