Summary: Recent findings have demonstrated prominent temporal influence of antecedent self-reported symptoms of hyperarousal upon other symptom clusters in emergent posttraumatic stress disorder (PTSD, Schell, Marshall, & Jaycox, 2004). However, the role of hyperarousal in the course of chronic PTSD over time is currently unknown. The present study examined the temporal role of hyperarousal influence upon the other diagnostic symptom clusters in chronic combat-related PTSD. Analysis of longitudinal data examined the influence of prior hyperarousal upon subsequent reexperiencing, emotional numbing, and avoidance symptoms. Data consisted of biweekly telephone assessments of PTSD symptoms and concurrent life stressors administered over a two-year period to male Vietnam theatre veterans (N = 34) who had been diagnosed with current or lifetime PTSD. Data analyses were conducted using multilevel random coefficients regression (HLM, Version 5.01, Raudenbush, Bryk, Cheong, & Congdon, 2001). Chronic symptom fluctuation was modeled using cross-lag autoregression, and symptom change in response to stressful world events was examined using logarithmic growth modeling. Within-subject analyses demonstrated that antecedent hyperarousal was the only PTSD symptom cluster to directly influence all other subsequent clusters across both two- and four-week time intervals, controlling for autoregressive effects. Similar to recent findings in emergent PTSD, hyperarousal demonstrated greatest temporal influence relative to the other symptom clusters across four-week intervals; hyperarousal predicted, and was not predicted by, any of the other symptom clusters. However, hyperarousal did not demonstrate the strongest temporal influence across two-week intervals; instead, both prior hyperarousal and prior emotional numbing demonstrated equal influence upon later reexperiencing and avoidance symptoms. Prior individual life stressors directly influenced only later hyperarousal symptoms and did not influence any other subsequent PTSD symptom group. Preliminary examination of between-individual differences in these within-individual relationships indicated that prior hyperarousal more strongly predicted the other subsequent symptom clusters for individuals with higher levels of negative affectivity. Taken together, these results indicate that the influence of hyperarousal may play a key role in mechanisms that maintain chronic PTSD over time.