As the cost of health care rises governments everywhere are examining how on-line services can replace or augment face-to-face services. Consequently, many health bodies are establishing on-line health forums where patients can share ideas with, or solicit information from, both other patients and health professionals. In the wake of this trend, many on-line forums have arisen which do not have the imprimatur of official government services but are run and managed by private individuals sharing experiences outside of the patient-clinician channel. This phenomenon creates risks and challenges for users who need to evaluate the credibility of unknown and often anonymous contributors to these forums. This paper examines how users assess the credibility of the information in these forums. Five criteria were discovered in the first stage of the work. We then quantitatively tested the relationship between those criteria based on two types of information. Our analysis shows that different criteria are used by participants in online health forums for scientific information and experiential information. We used these novel findings to develop a model for how information credibility is assessed in online health forums. These findings provide important lessons for health promotion bodies considering how to encourage the sharing of valuable health information on-line as well as guidelines for improved tools for health self-management.