Whereas visual hallucinations are often found among patients with Parkinson's disease, the occurrence of auditory hallucinations has never been systematically documented. The occurrence, past and present, of auditory hallucinations has been studied in 121consecutive patients with Parkinson's disease attending a movement disorders clinic. The cognitive state was evaluated using the short mental test (SMT). Hallucinations were reported for 45patients (37%); 35 (29%) had only visual hallucinations and 10(8%) both visual and auditory hallucinations. No patient reported auditory hallucinations unaccompanied by visual hallucinations. The auditory hallucinations occurred repeatedly, consisting of human voices. They were non-imperative (n=9), non-paranoid (n=9), and often incomprehensible (n=5). They were not obviously influenced by the patients' age, duration of disease, or treatment with levodopa. Cognitive impairment was more common among hallucinating patients (64%, 50%, and 25% among patients with visual hallucinations, auditory hallucinations, and non-hallucinating parkinsonian patients respectively). Depression necessitating antidepressants was present in five of 10 and other psychotic features in six patients with auditory hallucinations. It is concluded that auditory hallucinations occur in Parkinson's disease, particularly in patients who also have visual hallucinations and are cognitively impaired.