Urinary tract masses.

Disorders of the urinary tract account for the majority of abdominal masses in infants. The most common causes of a flank mass in a neonate are a hydronephrotic kidney followed by a multicystic kidney. An increasing number of disorders resulting in hydronephrosis are diagnosed by prenatal ultrasound scan, and their natural history continues to be elucidated. This review will examine the common anatomic and functional causes of fetal and neonatal hydronephrosis as well as the diagnosis and treatment of solid and cystic urinary tract masses.