To understand how a norm of self-disclosure forms and is adhered to in a synchronous computer-mediated discussion, participants discussed the stigma of mental illness. The transcripts of the discussion were coded for the number of self-disclosures, the number of statements supportive of self-disclosure, and the number of statements supportive of non-self-disclosure. The results showed that the number of self-disclosing statements increased over time, although not in a linear fashion, as did the number of statements supportive of self-disclosure. However, the number of statements supportive of non-self-disclosures decreased over time. These results suggest that once a norm of self-disclosure forms, it is reinforced by statements supportive of self-disclosures but not of non-self-disclosures. The results are discussed in the context of self-disclosure reciprocity and the social identity model of deindividuation effects (SIDE).