Renal masses in the neonate.

Abdominal masses are frequent in newborn infants, two thirds being renal in origin. After thorough palpation of the abdomen, cystic masses must be differentiated from solid masses by ultrasound examination. The common benign cystic masses include hydronephrosis, multicystic dysplasia, polycystic disease, and adrenal hemorrhage. Solid masses include renal vein thrombosis, a renal ectopic kidney, or horseshoe kidneys. Occasionally, a renal mass may be malignant and correspond to congenital mesoblastic nephroma, Wilms' tumor, or fetal hamartoma. Whatever the nature of the renal mass, early intervention may save the kidney or the patient.