Psychiatric symptoms typically cut across traditional diagnostic categories. In order to devise individually-tailored treatments, there is a need to identify the basic mechanisms that underlie these symptoms. Behavioral and neuro-economics methods provide a framework for studying these potential mechanisms. We utilized this framework to examine aspects of trauma-related symptomatology and its potential link to individual uncertainty attitudes. We distinguish between attitudes towards uncertain outcomes with known (“risk”) and unknown (“ambiguity”) probabilities, and between attitudes towards uncertain gains and uncertain losses. fMRI data were obtained from 57 combat veterans (30 with PTSD, and 27 without PTSD) who made choices involving risky and ambiguous options, which were used to estimate risk and ambiguity attitudes in the gain and loss domains. Veterans with PTSD were more averse to ambiguity, but not risk, compared to veterans without PTSD, when making choices between possible losses, but not gains. The degree of aversion was associated with anxious arousal symptoms, as well as with the degree of combat exposure. A whole brain analysis indicated association between activation in specific brain areas implicated in decision-making and severity of PTSD. Moreover, ambiguity attitudes fully mediated the association between combat exposure and anxious arousal symptoms. These results provide a foundation for the causal association between ambiguity attitudes and trauma-related symptoms, as well as etiology of the neural underpinnings of these behavioral outcomes. Results demonstrate the potential of neuroeconomic and behavioral economic techniques for devising objective and incentive-compatible diagnostic tools, and investigating the etiology of psychiatric disorders.