Hydronephrosis is a build-up of urine inside the kidney, usually because the drainage pipe is blocked or too narrow. Pressure stretches the kidney like a water balloon and sends quiet but clear warnings.

Flank pain is the headline. It feels like a deep ache just below the ribs or a sudden cramp that can shoot into the groin when the blockage shifts.

Urinary changes follow. You may notice less urine, a weak stream, or the need to go more often with only small amounts coming out.

Blood in the urine can appear. The color may be light pink or cola-brown, and it often shows up without any pain.

Nausea and vomiting can join in. The gut shares nerves with the kidney, so stretching the kidney can trigger a sick stomach.

Some people feel a heavy, swollen side. Pants fit tighter on one side, or you can press on the flank and feel a dull slosh.

If infection sets in, fever, burning urine, and chills arrive—an emergency signal that the stagnant urine has turned into a breeding ground for bacteria.