We focused in this study on how the private experience of pain is made public through online discourse by sufferers of endometriosis. Empirically, we analyse two highly active endometriosis communities on the online social platform Reddit. Drawing on a mixed-methods design, we leverage large-scale social data, and a combination of computational and interpretive approaches for text analysis to study the role and shape of interactions relating to ‘pain’ for the formation of epistemic community online around endometriosis. The dataset, consisting of 70,817 forum posts and comments, was collected in May of 2021. Our study shows how pain becomes meaningful for endometriosis sufferers in relation to a multidimensional discursive space of words and concepts that are used to express it. Pain was frequently disguised, underplayed or hidden altogether, from fears of misunderstanding, medical dismissal, and embarrassment. Clearly, peer validation can be found in the relative anonymity of Reddit discussions. While the experience of pain is individual and subjective, when communities share similar experiences this reinforces patient ownership of the pain, which in turn supports the epistemic authority of the patient collective. A detailed understanding of how and why pain is discussed in online spaces has much to contribute more broadly to discussions of experiential collective knowledge production among individuals with endometriosis and other chronic illnesses.