Postnatal regression of prenatally or neonatally detected multicystic dysplastic kidney disease has been widely documented. However, renal cysts can regress during gestation, although they usually become larger in utero. We present a case of prenatally detected multicystic dysplastic kidney with an atypical course. During the third trimester, unilateral multicystic renal lesions in the fetus first enlarged and later involuted; by the second postpartum year, the kidney had become hypoplastic and nonfunctional. Our case also shows that before birth, blood flow in the affected kidney, measured with Doppler imaging, was normal until the cysts involuted.