Objective: To evaluate the neuropsychological and psychiatric sequelae of unilateral posterior pallidotomy for treatment of PD. Methods: Patients with idiopathic PD completed baseline and 3- and 6-month assessments after random assignment to an immediate surgery (n = 17) or medical management (n = 16) group. Results: Compared with the medical management group, the immediate surgery group with single lesions centered on the posterior internal pallidum showed superior naming and response inhibition, better verbal recall at 6 months, but greater distractibility, a tendency toward lower phonemic fluency, and a transient (3 months' only) semantic fluency deficit. The group with left lesions had more neuropsychological deficits than the group with right lesions or the medical management group, although these occurred mainly at 3 (but not 6) months. At 6 months, the patients with left lesions showed better verbal memory retention than the patients with right lesions. On most measures, the pattern of individual clinical change did not differ as a function of surgery or lesion laterality, with the exception of a higher frequency of decline in phonemic fluency in the patients with left lesions at 6 months. Although psychiatric status did not change overall, a history of depression tended to increase the risk of a depressive episode following surgery. Conclusions: Well-targeted, uncomplicated, unilateral pallidotomy does not produce overall neuropsychological or psychiatric change, although there are subtle changes on specific measures sensitive to frontal lobe function.