Research has generally found a "dose relationship" between potentially traumatic events (PTEs) and the likelihood of developing PTSD, with greater number of events associated with greater likelihood. Most of these studies have been cross-sectional, however. A recent prospective study (Breslau, Peterson, & Schultz, 2008) found that PTSD response to prior potentially traumatic event (PTE) exposure, rather than prior exposure itself, acts as a risk factor for PTSD in response to subsequent PTE; however, this analysis combined many different types of events, and the unique contribution of specific events (e.g., assault) that may be associated with differential risk of PTSD was indeterminable. The present study examined the effects of cumulative PTE exposure prospectively using a two-wave design in the National Survey of Adolescents (N = 1703). History of assault and witnessing serious violence were the focal PTEs examined. Wave I assault without PTSD was found to predict PTSD at Wave II following exposure to new assault or witnessed violence; however, among those without prior PTSD, Wave I witnessed violence did not increase risk of subsequent PTSD following exposure.