Introduction. A number of reports have identified significant personality differences in patients with Parkinson's disease (PD) when compared with age-matched controls. We hypothesised that these differences may be related to impairment in prefrontal inhibitory functions resulting in the expression of new "Machiavellian" personality traits. Methods. Thirty-five patients with PD and 17 age-matched controls were assessed with a set of neuropsychologic, personality and mood tests as well as the Mach IV scale, which measures a set of "Machiavellian" personality characteristics. Results. PD patients with elevated Machiavellian traits ("high Machs") were selectively impaired on tests of prefrontal function relative to "low Mach" patients. In addition, while high Machs did not differ from low Machs in terms of age, educational level, Hoehn-Yahr stage, mood function, or Mini Mental State Exam score, they indicated greater willingness to affiliate with a fictional Machiavellian character and scored significantly lower on the "cooperativeness" and "self-directedness" subscales of the Cloninger Temperament and Character Inventory. Conclusions. We suggest that (1) PD patients with frontal impairment are vulnerable to dramatic personality change, and (2) the frontal lobes are required for maintenance of prosocial personality traits.