We describe a case of megacalycosis in a newborn female with a probable antenatal ultrasonographic diagnosis of left multicystic dysplastic kidney. Uroradiology, ultrasonography and nuclear medicine studies were performed and diagnosis was established by excretory urogram. We have found only one previously published case of megacalycosis in which diagnosis was made during the postnatal study of antenatal hydronephrosis. Megacalycosis should be included in the differential diagnosis of prenatal hydronephrosis.