Metanephric adenoma (MA) is a renal tumor that is rarely diagnosed in children. Although it is considered benign and to have a good prognosis, the diagnosis of MA is challenging because of histopathologic and radiologic similarities to Wilms tumor. In this case report, we present a 6-year-old girl, with a renal mass and right hemihypertrophy, who was previously diagnosed as Wilms tumor on a fine-needle biopsy and diagnosed as MA after nephroureterectomy. The differentiation between Wilms tumor and metanephric adenoma is also discussed.