Congenital iris ectropion has recently been added to the spectrum of neural-crest-derived anterior segment dysgenesis syndromes. Major features include a nonprogressive ectropion of the iris pigment epithelium, a glassy smooth cryptless iris surface, a high iris insertion, dysgenesis of the drainage angle, glaucoma, and in many cases, ipsilateral ptosis. Anterior segment dysgenesis syndromes have been subdivided into disorders of neural crest cell migration, proliferation, or differentiation. The congenital iris ectropion syndrome does not clearly fit into this classification. A new classification based on a theory of developmental arrest is presented which is more consistent with current knowledge of embryologic development and with recent clinical and histopathologic findings. The new classification links the congenital iris ectropion syndrome with the Axenfeld-Rieger spectrum but separates it from classic congenital glaucoma and the irido-corneal endothelial (ICE) syndromes. In addition, a histopathologically-supported etiologic theory for congenital iris ectropion is presented that supports the new classification.