Mesonephric adenocarcinoma of the bladder may be the malignant counterpart of nephrogenic adenoma. We report the third known case of this invasive, well differentiated, tubular neoplasm resembling nephrogenic adenoma. The invasive potential of nephrogenic neoplasms has altered our approach to the management of these lesions. Nephrogenic adenoma and mesonephric adenocarcinoma appear cytologically similar on a superficial bladder biopsy. The latter is excluded by deeper bladder biopsies. Muscular invasion may indicate a more aggressive behavior and may require radical cystectomy for cure.