Categorization of dot patterns is a frequently used paradigm in the behavioral study of natural categorization. To determine the human brain regions involved in categorization, we used Positron Emission Tomography to compare regional Cerebral Blood Flow patterns in two tasks employing patterns that consisted of nine dots. In the categorization task, subjects categorized novel exemplars of two categories, generated by distorting two prototypes, and other random dot patterns. In the control task, subjects judged the position of similarly distorted patterns. Each task was presented at two matched levels of difficulty. Fixation of the fixation target served as baseline condition. The categorization task differentially activated the orbitofrontal cortex and two dorsolateral prefrontal regions. These three prefrontal regions were equally weakly active in the position discrimination task and the baseline condition. The intraparietal sulcus was activated in both tasks, albeit significantly less in the position discrimination than in the categorization task. A similar activation pattern was present in the neostriatum. Task difficulty had no effect. These functional imaging results show that the dot-pattern categorization task strongly engages prefrontal and parietal cortical areas. The activation of prefrontal cortex during visual categorization in humans agrees with the recent finding of category-related responses in macaque prefrontal neurons.