Phonemic fluency impairments have been reported as one of the most consistent cognitive declines following pallidotomy for the treatment of Parkinson's disease. In 39 pallidotomy patients, we evaluated two dissociable processes involved in phonemic fluency: (1) clustering, the ability to generate words within clusters or subcategories, which has been related to intact temporal lobe functioning; and (2) switching, the ability to shift between clusters, which has been related to frontostriatal integrity. We also correlated the location of the lesion in the pallidum and adjacent structures with the effect on phonemic fluency. Pallidotomy patients generated significantly fewer words on phonemic fluency for up to a year following surgery, with a decline in the total number of switches made, rather than a decline in the number of clusters generated. Patients whose lesions on postoperative neuroimaging fell entirely or mostly within the GPi performed worse on switching than patients whose lesions fell half outside of the GPi, either within the GPe or the internal capsule. However, no significant differences were found for lesion location 6 months following surgery. The findings support the hypothesis that the GPi plays a role in cognitive functioning.