A 33-year-old man presented to the hospital with a complaint of sudden gross hematuria. He had no associated pain or additional complaints. His laboratory workup revealed normal levels of hemoglobin and tumor markers (CEA, CA19-9, NSE, SCC, sIL-2R, and IgG4). Atypical urothelial cells were not detected in the urinary cytological test. In a cystoscopic examination, gross hematuria from the left ureteral orifice was found, although no apparent abnormalities were observed in bladder mucosa. In contrast, enhanced CT revealed a 35-mm mass with slight enhancement, which significantly compressed the left renal vein. Collateral vessels were not apparent between renal parenchyma and inferior vena cava. Abdominal enhanced magnetic resonance imaging identified a low signal intensity of tumor at T1 as well as T2-weighted images with a slight enhancement and an almost normal intensity in diffusion weighted image suggesting a benign tumor. A CT-guided needle biopsy was not performed because the tumor was encroaching the renal vein, thus having a possible risk of hemorrhage. After obtaining the informed consent concerning surgery and subsequent publication, the patient underwent open tumor resection through retroperitoneal approach for easy extension of the resecting area in case the frozen section identified malignancy. During surgery, a yellowish, non-necrotic tumor compressing the renal vein was identified corresponding to the CT findings. The result of the intraoperative rapid pathological analysis suggested a benign tumor. Therefore, the left kidney was spared and the surgery was completed. Pathological examination identified the mass was directly surrounded by adipose tissue, lacking a distinct capsule, and was composed of adrenocortical-like cells. Medullary cells were not observed. The final pathological diagnosis was adrenocortical adenoma. According to the Weiss criteria, the estimated malignant potential of the tumor was low, with only one of the nine criteria met, which was clear cells comprising ≤25% of the tumor. Accordingly, the tumor was diagnosed as benign. The hematuria improved immediately after surgery, and no evidence of tumor recurrence was found during the 2-year follow-up, supporting the benign nature of the tumor. To further investigate the etiology and hormonal function of the mass, we performed immunohistochemical analysis of key steroidogenic enzymes, as previously reported:, 3βHSD, CYP11B2, CYP17, and CYP11B1. The PCA per TA of each stained section was measured by the Color Deconvolution software and the ImageJ software. The PCA/TA ratio of 3βHSD and CYP11B1 was 39.4% and 93.4%, respectively. The latter indicated that the tissue was of an adrenocortical origin. The PCA/TA of CYP17 was 10.0%, suggesting that some cells might have produced cortisol. The CYP11B2 staining result was positive only in a few cells (0.3%), indicating that the mass unlikely produced aldosterone. After removal of the tumor, the narrowing of left renal vein disappeared in the CT image. At present, more than 2 years after the operation, there is neither recurrence nor even microscopic hematuria.