The theory of müllerianosis predicts that embryonic müllerian tissue, misplaced during organogenesis, results in the formation of 4 benign müllerian diseases-developmental adenomyosis, endometriosis, endosalpingiosis, and endocervicosis-(developmental müllerian diseases) that will be identified in human female fetuses, infants, children, adolescents, and adults. Direct evidence is presented to support the existence of developmental adenomyosis, developmental endometriosis, and developmental endocervicosis in human female fetuses along with strong circumstantial evidence supporting the existence of all 4 developmental müllerian diseases in human female infants, children, adolescents, and adults. This evidence throws light upon the pathogenesis of rare müllerian lesions whose pathogenesis remains inexplicable by classical and modern theories. Furthermore, this research has scientific and clinical relevance: scientific relevance because it opens up a new field of comparative research-the 4 developmental müllerian diseases complement the 4 acquired müllerian diseases; clinical relevance because it identifies rare müllerian diseases curable by complete surgical excision.