Various lines of evidence suggest that the striatum is implicated in cognitive flexibility. The neuropsychological evidence has, for the most part, been based on research with patients with Parkinson's disease, which is accompanied by chemical disruption of both the striatum and the prefrontal cortex. The present study examined this issue by testing patients with focal lesions of the striatum on a task measuring two forms of cognitive switching. Patients with striatal, but not frontal lobe lesions, were impaired in switching between concrete sensory stimuli. By contrast, both patient groups were unimpaired when switching between abstract task rules relative to baseline nonswitch trials. These results reveal a dissociation between two distinct forms of cognitive f lexibility, providing converging evidence for a role of the striatum in f lexible control functions associated with the selection of behaviorally relevant stimuli.