Background: Pathologic gamblers are known to have abnormal neural responses associated with experiencing monetary wins and losses. However, neural responsiveness during reward and loss expectations in pathologic gamblers has not yet been investigated. Methods: We used a functional magnetic resonance imaging paradigm that allowed us to investigate the dissociable reward- and loss-related expectancies with various probabilities of winning or losing different amounts of money in 15 patients with problem gambling (PRGs) and 16 healthy control subjects (HCs). Results: Compared with HCs, PRGs showed stronger activation in the bilateral ventral striatum to 5 euro than to 1 euro trials. PRGs also showed more activation of the bilateral ventral striatum and left orbitofrontal cortex associated with gain-related expected value than HCs. In addition, regression analyses indicated a highly significant negative correlation between gambling severity scores and right amygdala activation associated with gain-related expected value coding. There were no group differences in brain activation for loss-related expected value. Conclusions: PRGs show higher activity in the reward system during reward expectation than HCs, whereas we observed no difference between PRGs and HC in the loss value system. Furthermore, the negative relation between gambling severity and amygdala activation in gain expected value coding suggests that more severe PRGs are less likely to be risk aversive during gambling. Our study provides evidence that PRGs are characterized by abnormally increased reward expectancy coding, which may render them overoptimistic with regard to gambling outcomes.