(See also Chapters 136 and 137.) In most clinical series, the majority of neonatal abdominal masses arise from the kidney. They may be unilateral or bilateral, solid or cystic. After physical examination, ultrasonography should be obtained to define whether the mass is solid or cystic, to determine the presence or absence of normal kidneys, and to assess for other intra-abdominal abnormalities. In selected instances, additional procedures such as renal scan, CT, retrograde pyelography, venography, and arteriography may be needed to define the pathology and plan appropriate therapy.