Online support groups are common sources of both health information and social support. To augment existing qualitative understandings of member roles and identities in OSGs, this article presents a corpus-based investigation of shifts in member lexicogrammatical and discourse-semantic choices in a bipolar disorder OSG. In total, 8.4 million words in 57,000 posts were transformed into a structured, grammatically annotated corpus and investigated using systemic functional linguistics as a theoretical framework, focusing on interpersonal and experiential meanings. The findings of mood and (...) transitivity analyses show marked differences between new and veteran members’ language choices over the course of membership in the OSG: particularly, in how veteran members provide advice, and in how new and veteran members ascribe and attribute bipolar disorder to both the self and others. Discussion addresses the affordances and limitations of corpus linguistics and SFL as strategies for providing quantitative support for key claims of earlier research in this field. (shrink)