Despite measurable benefits of childhood vaccines, mothers with high levels of social privilege are increasingly refusing some or all vaccines for their children. These mothers are often clustered geographically or networked socially, providing information, emotional support, and validation for each other. Mothers who reject vaccines may face disapproval from others, criticism in popular culture, negative interactions with healthcare providers, and conflicts with people they know, which serve to stigmatize them. This article uses qualitative data from in-depth interviews with parents who reject vaccines, ethnographic observations, and analyses of online discussions to examine the role of social capital in networks of vaccine-refusing mothers. Specifically, this article explores how mothers provide each other information critical of vaccines, encourage a sense of one's self as empowered to question social expectations around vaccination, provide strategies for managing stigma that results from refusing vaccines, and define a sense of obligation to extend social capital to other mothers. In examining these strategies and tensions, we see how social capital can powerfully support subcultural norms that contradict broader social norms and provide sources of social support. Even as these forces are experienced as positive, they work in ways that actively undermine community health, particularly for those who are the most socially vulnerable to negative health outcomes from infection.