A case is presented of gross hematuria and a large polypoid renal pelvic mass in a 15-month-old boy. Operation revealed a Wilms tumor with limited parenchymal involvement and rupture into the collecting system. This rare presentation of nephroblastoma and its differentiation from other rare benign and malignant renal pelvic lesions occurring in the pediatric patient are discussed. The advantages and disadvantages of radical and conservative surgical approaches to these rare lesions are reviewed. Preoperative cystoscopy and retrograde pyelography are mandatory in all patients with renal tumors presenting with gross hematuria, regardless of age. Radical nephrectomy with total ureterectomy is recommended for Wilms tumor with extension into the collecting system.