Patients with unilateral frontal- or temporal-lobe excisions and control subjects performed a cognitive risk-taking task in which target items (line-drawings or words) had to be guessed on the basis of partial-information clues. For each item, subjects chose to guess with either one, two, three, or all four clues, for a possible reward of 30, 20, 10, or 5 points, respectively. Two clue-presentation conditions were used to allow the differentiation of risk-taking from impulsivity. Whereas no group obtained high risk-taking scores, patients with frontal-lobe lesions demonstrated impulsive behaviour when manual responses were required. Only the left frontal-lobe group was impaired at solving the clues.