Objective: Attention-deficit/hyperactivity disorder (ADHD) has been linked to deficits in the dopaminergic reward-processing circuitry; yet, existing evidence is limited, and the influence of genetic variation affecting dopamine signaling remains unknown. We investigated striatal responsivity to rewards in ADHD combined type (ADHD-CT) using functional magnetic resonance imaging (fMRI), and whether it is modulated by variation in the dopamine transporter gene ("DAT1"). Method: We tested 29 male adolescents with ADHD-CT and 30 age-, handedness-, and gender-matched healthy controls who were selected for "DAT1"[subscript 10/6] haplotype dosage. Based on previous research, we focused our analysis on the ventral striatum and the caudate nucleus. Results: Three main findings emerged. First, male adolescents with ADHD-CT did not differ from controls in terms of blood oxygen-level dependent (BOLD) fMRI response to reward-predicting cues (gain or loss-avoidance) in the ventral striatum. Second, male adolescents with ADHD-CT showed a relative increase, compared with controls, in the striatal BOLD response to successful outcomes. Third, "DAT1"[subscript 10/6] dosage differentially modulated neural activation to reward-predicting cues in the caudate nucleus in the ADHD-CT and control groups. Conclusions: The findings challenge the idea of a deficit in anticipation-related activation in the ventral striatum in male adolescents with ADHD-CT, while suggesting that the processing of reward outcomes is dysfunctional, consistent with a recent neurobiological model of the disorder. Preliminary evidence suggests that polymorphic variations in genes affecting dopamine signaling need to be taken into consideration when investigating reward-related deficits in ADHD-CT. (Contains 10 tables and 4 figures.)