Based on emotional processing theory, preexisting negative cognitions may contribute to the development of posttraumatic stress disorder (PTSD) symptoms. The present study prospectively examined the association between preexisting PTSD-related cognitions and subsequent acute PTSD symptoms, and the potential mediators of this association. We also compared the effect of preexisting depressive cognitions and preexisting PTSD-related cognitions on PTSD symptoms. In the current study, 810 Taiwanese undergraduates completed a baseline survey (T1), of which 73.1% (n = 592) participated in a second survey two months later (T2). Of those who completed both surveys, 97 experienced a trauma at least one week before T2; this group comprised the final sample. Hierarchical regression showed that preexisting PTSD-related cognitions (β = .38, p < .001, sr2 = .117), but not preexisting depressive cognitions (β = .11, p = .315, sr2 = .011), were a significant and substantial predictor of acute PTSD symptoms after we controlled for established pretrauma risk factors (i.e., gender, prior trauma, and prior psychological problems). Multiple mediation analysis revealed that negative appraisal of symptoms (a1 b1 = 0.90, 95% CI [0.16, 2.18], PM = .251) and trauma-related rumination (a3 b3 = 1.23, 95% CI [0.23, 2.86], PM = .341), but not trauma memory disorganization (a2 b2 = 0.65, 95% CI [-0.17, 1.92], PM = .182), significantly mediated between preexisting PTSD-related cognitions and acute PTSD symptoms. Our findings highlight the role of preexisting negative cognitions in acute PTSD symptomatology. The development of PTSD symptoms is likely determined by the interaction of risk factors before and after trauma. [ABSTRACT FROM AUTHOR]