Executive deficits of problem solving and concept formation have been associated with frontal lobe dysfunction. Here we describe a new clinical test of concept formation based on the parlor game, Twenty Questions. The Twenty Questions Test requires examinees to ask the fewest number of yes/no questions possible in order to identify a target item from an array of 30 line drawings. The items belong to a number of categories and subcategories that exist in a hierarchical, semantic structure. Patients with focal prefrontal lesions asked significantly more questions than controls in their attempt to guess the target items and sometimes exhausted the 20-question limit. Qualitative analyses revealed that patients tended to use ineffective categorization strategies, for example, relying exclusively on questions that referred to single items. Taken together with previous findings, we conclude that prefrontal cortex supports the on-line organization and conceptualization of category exemplars in concept-formation tasks.