A premature infant with segmental hypoplasia of one kidney is reported. Since 1929 when Ask-Upmark first reported this renal anomaly and its association with hypertension, the nature of the lesion has remained in doubt. The question whether it is acquired post-natally or whether it is a developmental mal-formation has not been resolved. The case which we now report clearly documents the prenatal development of the lesion as well as its association with arrested nephrogenesis and the presence of abnormal vasculature. The cause of these changes is not clear, but it is suggested that ureteral reflux was present.