Seventy-seven patients with different cerebral lesions were tested on a rule-detection task where the stimuli were designed in such a way as to minimize the activation of pre-existing schemata. Patients with lesions involving the frontal lobes were poorer at achieving set than patients with lesions elsewhere. In addition, the anteriorly-lesioned group showed a greater tendency to guess and were more likely to abandon a correct rule once it had been attained, but there were no differences between the groups in incidence of perseverative responses. Various plausible explanations of these results are examined, with the most favoured account suggesting that anterior patients show an exaggerated willingness to adopt bizarre hypotheses.