Patients suffering from chronic diseases constantly face the need to learn how to self-manage their health condition and often turn to the Internet to seek help and support from health professionals and laypeople. Internet-based interventions can be an effective tool for patients' education and have been shown to positively impact patients' health outcomes. Focusing on the process leading from Internet usage to health outcomes, this paper reports a model-driven evaluation of a specific Internet-based intervention for patients affected by fibromyalgia syndrome. The evaluation is based on a cross-sectional survey of 209 fibromyalgia patients who were involved in the Internet intervention. Analyses were conducted using a structural equation modeling approach. The analysis mostly confirms the assumed theoretical model with minor and theoretically sound modifications. Results show that the usage of certain tools of the application impacts patients' health knowledge, which in turn impacts self-management. Improvements in self-management ultimately lower the impact of fibromyalgia syndrome. These results are discussed, alongside major implications for the evaluation of Internet-based interventions.