Comorbidity has been used extensively to explain the numerous co-occurring psychiatric syndromes accompanying chronic posttraumatic stress disorder (PTSD). A cascade model is proposed as an alternative to comorbidity for the pathogenesis and clinical course of the condition. This model allows for a dynamic, integrated conceptualization of disease progression in PTSD. Findings in the clinical, epidemiological, neurobiological, and psychosocial literature which might support this model are described. Conceptual and heuristic difficulties and/or potential objections to the model are also examined. Finally, diagnostic and treatment implications as well as potential research applications of the model are discussed. © 1999 Taylor & Francis Group, LLC. All rights reserved.