Our study evaluated the relationship between positive affect (PA) and negative affect (NA) traits on the development of posttraumatic stress symptoms (PTSS) among peacekeepers. A longitudinal study with 138 army personnel deployed to a peacekeeping mission in Haiti was conducted. An instrument for measuring PA and NA traits was used before deployment. PTSS, indexed by posttraumatic stress disorder Checklist--Military Version (PCL-M) and frequency of stressful situations were measured after return. Regression analysis showed that both NA and number of stressful situations contributed toward increasing PCL-M scores (Adjusted R = 0.25; p < 0.001). We also found that NA traits interact with intensively stressful situations enhancing the occurrence of PTSS (Adjusted R = 0.32; p < 0.001). These findings suggest that NA traits are an important predictor for PTSS among peacekeepers and also worsen the consequences of being exposed to stressful situations.