Introduction: Endometriosis is a debilitating condition where endometrial-like tissue grows outside the uterus and rarely in distant sites such as the lungs, heart and brain. Symptoms are severe chronic pain, bladder/bowel problems, painful sex and infertility. Differences in endometrium between women with and without endometriosis have not been constructive, and there is still paucity in its pathogenesis and non-invasive diagnostic test, leading to delayed diagnosis (~10 years) and effective treatment. Menstrual fluid (MF) gives a peak into endometrial health. Could MF extracellular vesicles (EVs) point to the pathogenesis and a non-invasive diagnostic tool for endometriosis? Aims: To identify differences in protein-cargo in MF-EVs from women with and without endometriosis, define their functional contribution to mesothelial niche cells for lesion establishment and determine whether MF-sEV can be a non-invasive endometriosis diagnostic tool. Methods: MF was collected from women with and without endometriosis on day 2 of menses and EVs isolated using differential centrifugation. MF-EVs were identified by size, morphology and protein markers and subjected to TMT- quantitative proteomics assay. Differentially expressed proteins were analysed. Ctrl and Endo MF-sEV were co-cultured with mesothelial cells to assess for uptake and changes in the adhesive and junctional proteins. Results: Spherical-shaped MF-EVs were identified with mean diameters of 121.9 nm (Endo) and 123.8 nm (Ctrl), expressing TSG101, Alix and Syntenin-1 sEV proteins. MF-sEV proteins originated from endometrial cells: stem cells, leucocytes, and endothelial and smooth muscle cells, validating the endometrial origin of the MF-EVs. 77% of identified proteins were differentially expressed. In endo-MF-sEV, proteins regulating HGF receptor (anoikis), cellular senescence and multiple apoptotic, nitrogen compound metabolic processes and TRP channels signalling (immunity and contraction regulation) were decreased, potentially promoting lesion establishment, survival and pain. Similarly, antibacterial peptides (cathelicidin, mucin-1, lipocalin2 and serum amyloid A) were downregulated. The majority of proteins were significantly downregulated in endometriosis suggesting a dysregulation in the protein synthesis and cellular communication for homeostasis. Two-thirds of upregulated proteins were immunoglobulins indicating inflammatory pathogenesis. This was reflected by an increased Endo-MF-EVs uptake by mesothelial cells, with decreased scribble resulting in decreased cellular resistance and permeability. Conclusions: MF-EVs contain vital information indicative of endometriosis pathogenesis and may be the key to developing a simple and early diagnostic tool.