Brain dopamine neurons code rewarding environmental stimuli by releasing endogenous dopamine, a transmission signal that is important for reinforcement learning. Human reward-seeking gambling behavior, and especially pathological gambling, has been presumed to be modulated by brain dopamine. Methods: Striatal dopamine release was studied with [C]raclopride positron emission tomography (PET) during gambling with an ecologically valid slot machine gambling task. Twenty-four males with and without pathological gambling (DSM-IV) were scanned three times, and the effects of different gambling outcomes (high-reward and low-reward vs. control task) on dopamine release were evaluated. Results: Striatal dopamine was released in both groups during high-reward but also low-reward tasks. The dopamine release during the low-reward task was located in the associative part of the caudate nucleus. During the high-reward task, the effect was also seen in the ventral striatum and the magnitude of dopamine release was associated with parallel gambling "high". Furthermore, there was a positive correlation between dopamine release during the low-reward and the high-reward task. There was no general difference in the magnitude of dopamine release between pathological gamblers and controls. However, in pathological gamblers, dopamine release correlated positively with gambling symptom severity. Conclusions: Striatal dopamine is released during gambling irrespective of gambling outcome suggesting that the mere expectation/prediction of reward is sufficient to induce dopaminergic changes. Although dopamine release during slot machine gambling is comparable between healthy controls and pathological gamblers, greater gambling symptom severity is associated with greater dopaminergic responses. Thus, as the dopamine reward deficiency theory predicts blunted mesolimbic dopamine responses to gambling in addicted individuals, our results question the validity of the reward deficiency hypothesis in pathological gambling.