In the course of the ultrasound screening of 7057 newborns, 429 (6.1%) pyelectases were encountered (the pyelon was 5 mm or wider). Up to the termination of the study 91.7% of the newborns with pyelectasis could be controlled. In 341 (86.3%) out of the 395 controlled cases, pyelectasis disappeared without causing any disturbances. In 54 cases (13.7%) pyelectasis increased, or the considerable pyelectasis accompanied by calyectasis was not found to be reduced by the first control examinations. Vesicoureteral reflux was observed in 8 (14.8%) of these cases. Following a long-term observation period, all pyelectases disappeared or diminished considerably. Hydronephrosis was encountered in 23 newborns, 9 of them underwent surgery. The frequency of pyelonephritides was 1.4% during infancy of the newborns with pyelectasis. Pyelectasis of the newborn is considered a benign alteration. Its further examination is indicated if it increases and is accompanied by considerable calyectasis or ureterectasis.