Decision making is essential in everyday life. Though the importance of the mesial temporal structures in emotional processing and feedback learning is well established, decision making in patients after mesial temporal lobe surgery has rarely been studied. We studied 23 patients with mesial temporal lobe epilepsy after unilateral temporal lobe surgery and 26 healthy controls using two different gambling tasks, the Iowa Gambling Task (IGT) and the Probability-Associated Gambling (PAG) task. In the IGT, rules for gains and losses are implicit; in the PAG task, winning probabilities are well defined. Compared with healthy controls, patients performed poorly on the IGT (decisions under ambiguity). No group difference was found in the PAG task (decisions under risk). Results of the present investigation indicate that after mesial temporal lobe surgery, patients have difficulties in making decisions under ambiguity, whereas they decide advantageously when information about the decision situation is explicitly given.