Hallucinations occur fairly frequently in the course of the pharmacological treatment of Parkinson disease. Our aim in this study was to assess first the relation between hallucinations and mental deterioration and second the correlation between the perception disorder and the profile on the Minnesota Multiphasic Personality Inventory (MMPI). Of 304 parkinsonian subjects followed as outpatients at our center 27 (8.88%) had had hallucinations and 17 of these presented marked cognitive deficits (Mini Mental State less than 18) (62.9%) compared with 32 of the other 277 patients (11.5%) (X2 = 55.16, p less than 0.0001). A group of 9 patients who had had hallucinations and 10 controls who had not, all free from marked cognitive deficits, were assessed on the Luria-Nebraska Neuropsychological Battery (LNNB) and on the MMPI. The two groups did not differ significantly in respect of the LNNB but did in respect of the MMPI scales. On this evidence the frequency of mental deterioration is significantly higher in patients who have hallucinated. We discuss the meaning of the MMPI differences both from the biochemical angle and from that of personality factors. These data suggest that the MMPI might be useful for predicting hallucinations in undeteriorated parkinsonian patients on pharmacological therapy.