Imperforate hymen is a rare disorder that is usually discovered at the onset of menstruation. In the literature, secondary closure of the hymen has been reported in 2 cases, both of which occurred subsequent to surgical procedures that involved the hymen. We report an interesting case of the spontaneous formation of an imperforate hymen during pregnancy in the absence of previous surgical procedures. Electron microscopic findings indicate hymenal tissue reorganization. (c) 2005 Mosby, Inc. All rights reserved.