How is your son? Hydronephrosis or swelling of one kidney due to a backup of urine is a condition that usually occurs with an underlying condition such as acute or chronicunilateral obstructive uropathy, vesicoureteric reflux, or kidney stones. Treatment and prognosis depend on what is causing the kidney swelling. Treatment may include medical or putting a stent. Talk to your doctor about the diagnosis and results to ease any anxiety. Take care and do keep us posted.