To better understand participation in computer-mediated social support (CMSS) groups for breast cancer patients, this study examines two overarching questions of: 1) who are posters, lurkers, or non-users? and 2) what role do these different types of engagement play in explaining psychosocial health outcomes? This study incorporates the comprehensive model of information seeking and two competing models of social enhancement and social compensation, as well as the literature of lurking and posting behaviors in online groups to answer research questions. Our findings suggest that patterns of engagement in a CMSS group differed according to patients’ socio-demographic characteristics and psychosocial factors. In addition, we found that lurkers have a higher level of perceived functional well-being than posters at 3 months post-baseline. Theoretical and practical implications for effective online cancer support group campaigns are discussed.