Renal agenesis is generally thought to result from a lack of induction of the metanephric blastema by the ureteral bud, which may be secondary to ureteral bud maldevelopment and/or to a problem with the formation of the mesonephric duct. Multicystic dysplasia is thought to result from early ureteral obstruction as evidenced by the high frequency of associated proximal ureteral atresia. The recent obstetrical practice of large scale screening of large numbers of fetuses with sonography has resulted in a significant increase in the detection of this and other genitourinary anomalies. These findings have contributed to our understanding of the natural history of many of these malformations, resulting in ongoing reassessment and refinement of current management modalities. Thus, it is now well established that a significant number of multicystic dysplastic kidneys, initially detected in utero and followed postnatally, involute and disappear with time. We report the unequivocal presence of unilateral multicystic dysplasia detected during maternal sonography in 3 fetuses, which could not be seen on subsequent studies during the fetal or immediate postnatal period. To our knowledge, these cases constitute the first report of complete in utero regression of multicystic kidneys. This novel observation indicates that some cases of unilateral renal agenesis result from in utero regression of multicystic dysplastic kidneys. This phenomenon may explain the presence of an ipsilateral blind ending ureter found in some patients with unilateral renal agenesis. However, this infrequent observation does not explain all cases of renal agenesis. Thus, the etiology of renal agenesis remains multifactorial.