['The development of non-invasive diagnostic methods for endometriosis requires sensitive and disease specific biomarkers. Here, we describe the use of aspirated endometrial fluid from women with and without endometriosis as a novel biological sample for biomarker discovery.', 'Differential protein expression profiling of aspirates from women with early endometriosis (n = 14), advanced endometriosis (n = 32) and without evidence of the disease (n = 32) was assessed by two-dimensional gel electrophoresis (2-DE). A biomarker validation study was performed in an independent cohort (early endometriosis n = 6 and advanced endometriosis n = 14, controls n = 15).', 'The analysis resulted in the identification of 31 proteins showing statistically significant differences in expression. The proteins identified are related to cell signalling, cell death and cell movement, processes that may be involved in the onset and/or progression of endometriosis. The differences in expression observed for 14-3-3 (signal transduction) and moesin (cytoskeletal structure) were confirmed in an independent group of endometriosis patients.'] Is endometrial fluid a specific and non-invasive biological sample for protein biomarker identification in endometriosis?