This dissertation investigated the effectiveness of an Internet-based intervention for posttraumatic stress disorder (PTSD) resulting from involvement in a motor vehicle accident (MVA). MVA-related PTSD is a prevalent and potentially severe psychiatric disorder that is a costly and significant public health issue. While effective treatments have been developed for MVA-related PTSD, they tend to be fairly lengthy, and require a significant investment of time and money; moreover, they are not widely available. It was proposed that an Internet-based intervention could provide some of the benefits of face-to-face treatments in a less costly and more accessible format. The Internet-based intervention in the present study was based on a modified version of the cognitive-behavioral treatment protocol for MVA-PTSD developed by Hickling and Blanchard (1997). This is the first study of an Internet-based psychosocial intervention to rely entirely on a computer-based and therapist-free assessment and treatment protocol. Participants completed an online assessment, and those meeting eligibility requirements were randomly assigned to either a seven-week, Internet-based intervention (iTx), or a delayed treatment condition (dTx) with a 6-week waiting period followed by treatment. Although participant attrition was high, participants who completed treatment appeared to benefit from the Internet-based protocol.