Web-based self-management interventions (W-SMIs) are designed to help a large number of chronically ill people become more actively engaged in their health care. Despite the potential to engage more patients in self-managing their health, the use of W-SMIs by patients and their clinicians is low. Using a self-management conceptual model based on self-efficacy and health belief theories, the purpose of this study was to explore the effect of a W-SMI on the patient activation levels of participants with a variety of chronic diseases. Patient activation is a measure of a person's knowledge, problem-solving skills, and self-confidence in self-management capabilities. Participants included 201 adults with chronic diseases who were randomly placed in 2 groups in a controlled trial. Parametric statistical models ("t" test, ANOVA, and ANCOVA) were applied to draw inferences. The W-SMI demonstrated a significant, positive effect on patient activation. Intervention participants at the earliest stages of patient activation experienced significant improvement in activation levels compared to those at the most advanced stage. A strong relationship between W-SMI use and change in patient activation was found. Results indicated that a W-SMI can improve patient activation of individuals with various chronic diseases. Areas for future inquiry include validation of the results in different settings and exploration of the causal chain of W-SMIs, patient activation, and health outcomes. Results of this study could contribute to positive social change by increasing understanding of the effect of W-SMIs on developing and sustaining activated patients across broad populations of the chronically ill who are better able to self-manage their conditions and, therefore, more effectively maintain their health. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]