Studies using the Iowa gambling task (IGT) have shown that patients with Parkinson's disease (PD) make disadvantageous choices characterized by immediate large rewards and delayed larger punishments. These results can be interpreted in two ways: either PD patients are hypersensitive to immediate outcomes and/or insensitive to delayed consequences or PD patients are hypersensitive to rewards and/or insensitive to punishments. In this study, we used a modified IGT in which selection of cards from the disadvantageous decks leads to immediate, small punishments and delayed, smaller rewards and selection of cards from the advantageous decks leads to immediate, large punishments and delayed larger rewards. We then compared the results obtained using this modified IGT with those obtained using the original IGT. If the PD patients were hypersensitive to the immediate outcomes of decisions, they would make disadvantageous choices in both the original and the modified IGTs. Differences between the results of the original and modified tasks would indicate impairments in balancing reward and punishment. In our analysis, PD patients selected advantageous decks and gained as much as normal subjects during the modified IGT, but they selected disadvantageous decks during the original IGT. These results indicate that the decision-making difficulties of PD patients are caused by their inability to balance reward and punishment and their hypersensitivity to reward and/or insensitivity to punishment.