Until recently, adolescent endometriosis has been considered a rare, often transient, condition not particularly serious for the young patient. Against this background, and not without disagreement by some, we have pointed out that early onset endometriosis (EOE) may at times be severe, necessitating quick diagnosis and proper treatment (Brosens et al., 2013b, 2014). To substantiate this viewpoint, we carefully searched published research, including some early paediatric research, and identified peritoneal reflux from neonatal uterine bleeding (NUB) occurring in 3–5% of female neonates, as a biologically plausible and likely cause of EOE (Brosens and Benagiano, 2013, 2016; Brosens et al., 2013a; Gargett et al., 2014).