['Uterus didelphys with obstructed hemivagina (UDWOH) and ipsilateral renal agenesis is a distinct but rare entity.', 'To demonstrate the association between UDWOH and a multicystic dysplastic kidney (MCDK) in neonates. To demonstrate the usefulness of sonography with vaginal fluid instillation in the early and reliable diagnosis of this genital anomaly in girls with MCDK.', 'Our patients were five female neonates with a MCDK, which had been detected in utero. We performed sonography of the internal genitalia in all patients before and after filling the vagina with saline.', 'In all five girls, sonographic examination revealed uterus didelphys with obstruction of the vagina ipsilateral to the MCDK. In three girls, a dilated ureter originating from the MCDK and extending to the level of the vagina could be clearly demonstrated, with one of these including the ureteric insertion into the atretic vagina.'] Is uterus didelphys with unilateral vaginal atresia : multicystic dysplastic kidney the precursor of `` renal agenesis '' and the key to early diagnosis of this genital anomaly?