With the growing interest in the role of cognitions in PTSD, this prospective study examined the course and bi-directional relationship between post-trauma cognitions and symptoms of PTSD. A sample of Israeli combat veterans, including former prisoners of war, was assessed in 1991, and later followed up in 2003 and 2008. PTSD symptoms were measured at three time points. Cognitions concerning the self and the world were measured twice. Applying autoregressive cross-lagged (ARCL) modeling strategy, initial PTSD symptoms predicted subsequent negative cognitions but not vice versa. In addition, repeated measures design revealed that individuals with chronic PTSD symptoms had relatively negative cognitions that further amplified with time. More specifically, increasingly negative cognitions were documented among ex-prisoners of war. The main findings suggest that negative cognitions are fueled by PTSD and that in chronic PTSD there is an amplification of pathogenic outcomes over time. Discussion of the findings is in the context of current cognitive models of PTSD.