Endometriosis is a chronic gynaecological condition of uncertain aetiology characterised by menstrual irregularities. Several studies have previously identified a lengthy delay experienced by patients between the first onset of symptoms and eventual diagnosis. Various explanations have been advanced for the diagnostic delay, with both doctors and women being implicated. Such explanations include that doctors normalise women's menstrual pain and that women might delay in seeking medical advice because they have difficulty distinguishing between 'normal' and 'abnormal' menstruation. It has been suggested that the diagnostic delay could be reduced if women were trained in how to distinguish between 'normal' and 'abnormal' menstrual cycles. In this paper I argue that whilst these may be factors in the diagnostic delay, women's reluctance to disclose problems associated with their menstrual cycle may be a more significant and hitherto neglected factor. I argue women are reluctant to disclose menstrual irregularities because menstruation is a 'discrediting attribute' (Goffman, 1963) and disclosure renders women vulnerable to stigmatisation. Women actively conceal their menstrual irregularities through practices of the 'menstrual etiquette' (Laws, 1990) which involves the strategic concealment of menstrual problems. This argument is supported through an analysis of the experiences of 20 Australian women diagnosed with endometriosis. The ramifications of this analysis for chronic pain conditions more generally and for practical strategies designed to address the endometriosis diagnostic delay are considered.