Social support can significantly improve health outcomes for individuals living with disease, and online forums have emerged as an important vehicle for social support. Whereas research has focused on the delivery and use of social support, little is known about how these communities are sustained. We describe one sociotechnical mechanism that enables sustainable communities to provide social support to a large number of people. We focus upon thirteen disease-specific discussion forums hosted by the WebMD online health community. In these forums, small, densely connected cores of members who maintain strong relationships generate the majority of support for others. Through content analysis we find they provide informational support to a large number of more itinerant members, but provide one another with community support. Based on these observations, we describe a sociotechnical mechanism of online support that is distinct from nonsupport oriented communities, and has implications for the design of self-sustaining online support systems.