Metanephric adenoma is a recently described rare and benign renal neoplasm. Our patient, a 37-year-old woman, suffered from flank pain for five months and was found to have a renal mass. Ultrasound, computerized tomography and angiography findings were consistent with a hypovascular renal cyst. Wilms' tumor was the initial misdiagnosis, based on needle biopsy and aspiration cytology. A radical nephrectomy was performed. Histologically, the tumor was well defined and was composed of uniform small cells arranged in a solid, tubular or rosette-like pattern. The prognosis is good for metanephric adenoma. The tumor was first considered a benign counterpart of papillary carcinoma or Wilms' tumor; however, recent cytogenetic evidence suggested that the tumor might be related to papillary adenoma and papillary renal cell carcinoma. The clinical, radiologic, histologic and cytologic features presented here should help to promote the correct preoperative diagnosis and to avoid unnecessary aggressive treatment.