The basal ganglia and cerebellum have both been implicated in motor skill acquisition. Recent hypotheses concerning cognitive functions of the basal ganglia and cerebellum have emphasized that these subcortical structures may also contribute to nonmotor learning. To explore this issue, patients with Parkinson's disease (PD) and patients with cerebellar lesions (CB) were tested on two category-learning tasks. Identical stimulus displays were used for the two tasks, consisting of a reference line and target line. In the length task, the two categories were defined based on the length of the target line. In the distance task, the two categories were defined by the distance between the target and reference lines. Thus, both categories could be defined by a simple rule in which attention must be restricted to a single relevant dimension. Consistent with previous results, the patients with PD were impaired on both tasks compared with neurologically healthy controls. In contrast, the CB patients performed similar to the control participants. Model-based analyses indicate that the patients with PD were able to select the appropriate categorization rule, but that they adopted suboptimal category boundaries in both conditions and were more variable in the application of the selected rule. These results provide an important neuropsychological dissociation on a non-motor-learning task between the effects of basal ganglia and cerebellar lesions. Moreover, the modeling work suggests that at least part of the Parkinson patients' impairment on these tasks reflect a tendency to exhibit strong response biases.