E-patients are increasingly using the Internet to gain knowledge about medical conditions, thereby problematizing the biomedical assumption that patients are ‘lay’. The present paper addresses this development by investigating the epistemic identities of patients participating on an online health forum. Using poststructuralist discourse analysis to analyze a corpus of cardiology-related threads on an ‘Ask a Doctor’ forum, we compare how patients are discursively constructed by online professionals as ‘knowing’ or ‘not knowing’ with the online knowledge identities patients choose for themselves. (...) Analysis reveals a complex picture, with patients positioning themselves and being constructed as biomedical novices, as well as claiming the subject positions of experts challenging medical expertise. This paper provides a snapshot of an important social identity in transition, illustrates a procedure for comparing language use around imposed and self-appropriated identities, and considers discursive choice in relation to the metapragmatic matter of “sayability”. (shrink)