BACKGROUND Congenital imperforate hymen is an external urogenital anomaly that has a small differential diagnosis. With careful antenatal sonographic evaluation of the female perineum, identification of a thin protruding membrane consistent with imperforate hymen can be made. METHODS We report a case in which an imperforate hymen was diagnosed in a fetus at 25 weeks' gestation. It appeared as a thin membrane that distended the vagina and spread the labia majora because of retained secretions. An associated renal abnormality was present, representing either a right multicystic dysplastic kidney or cystic dysplasia. CONCLUSIONS Imperforate hymen and hydrocolpos can be diagnosed as early as the second trimester. Because of the additional renal anomaly in this case, it is suggested that the presence of prenatally diagnosed imperforate hymen warrants a careful survey of the remaining fetal anatomy to rule out associated abnormalities. The possibility of a familial occurrence should be considered, and the appropriate history should be obtained.