Collection of urine in the renal pelvis that results in dilatation of the renal pelvis and calyces. It is caused by obstruction of urine flow, nephrolithiasis, or vesicoureteral reflux. Signs and symptoms include flank pain, nausea, vomiting, fever, and dysuria. The disease is Monarch Disease Ontology id MONDO_0005510 (hydronephrosis).