Twelve fetuses with renal pelvis dilatation < 15 mm have been followed up until a year after birth. Dilatation cleared up in 59% of the cases within the last weeks of pregnancy and in 33% in the first months of life, while was asymptomatically detected in one case only a year after birth. A grade I and II vesico-ureteral reflux was diagnosed in two babies. Scheduled ultrasonographic examinations together with routine biochemical tests only are suggested by the kidney anatomical-functional normal parameters. Possible recurrent infections may be prevented through urine culture. Voiding cystourethrography is performed only when a recurrent infection is present because a reflux may be detected. Lacking a definite prognostic parameter, pre- and postnatal ultrasonographic follow-up is necessary although the little renal pelvis dilatation is to be considered negligible.