The authors describe three patients with episodes of abdominal pain where despite a negative finding in the urine sonography revealed dilatation of the hollow system. Urography proved stenosis of the pyeloureteral junction. The authors discuss the cause of abdominal pain. Consistent with data in the literature, the authors draw attention to pitfalls in the diagnosis of hydronephrosis. They recommend sonographic examination of children with abdominal pain, as a negative urinary finding does not rule out hydronephrosis. In this respect screening of hydronephrosis and other obstructive anomalies during the neonatal period is an advantage.