A 44-year-old man with gross hematuria and left flank pain visited our emergency department on July 14, 2022. The patient’s gross hematuria started 1 day before admission. In the morning of the day of visit, the hematuria became more severe and was accompanied with clots. Moreover, he experienced severe pain in the left flank. The patient was healthy and had a history of appendectomy 10 years earlier. The patient denied any history of tobacco or alcohol consumption. He had no medication history. His family history was unremarkable. At the emergency department, the patient’s initial vital signs were stable. His blood pressure level was 129/79 mmHg, pulse rate was 81 beats/min, and respiratory rate was 16 breaths/min. The patient’s body weight was 86 kg, and his height was 171.0 cm (body mass index = 29.4 kg/m2). There was no palpable mass around the left flank area. The patient showed left flank tenderness but no rebound tenderness. Laboratory serum tests were normal (hemoglobin level: 13.7 g/dL, creatinine level: 1.05 mg/dL, and calcium level: 9.1 mg/dL). Urinalysis revealed the presence of many red blood cells. Electrocardiogram indicated normal sinus rhythm. Initial kidney dynamic computed tomography (CT) (three phases of intravenous contrast enhancement) revealed an 8.8 cm × 6.1 cm heterogeneously enhancing mass in the upper to mid pole of the left kidney abutting the left adrenal gland and pancreas. No metastatic lesions were detected in the enhanced brain and chest CT scans. Follow-up CT 3 mo after radical nephrectomy showed no metastatic lesions. However, at 6 mo postoperatively, abdominal CT revealed a 12-mm enhancing nodule in the left lower peritoneum. 18F-fluorodeoxyglucose (FDG) positron emission tomography/CT was performed immediately, and the results were the same (mildly hypermetabolic nodule in the left lower peritoneum).