A 49-year-old female was incidentally found to have a left renal tumor during a medical check-up. The tumor was too small to be fully diagnosed using computed tomography (CT) or magnetic resonance imaging (MRI). Since it was small and showed a homogenous enhancement pattern on contrast-enhanced CT, which made it difficult for us to distinguish the malignancy of the tumor, we performed regular CT follow-up. On the fifth year of her regular follow-up, the tumor had grown apparently larger and showed a heterogenous enhancement pattern, which suggested a malignant tumor. Since the tumor was exophytic, we decided to perform a laparoscopic partial nephrectomy. The operation was performed without any serious complications, and her renal function remained unchanged. The histopathology of the tumor was leiomyoma. Here, we discuss the characteristics of this tumor and the role of immunohistopathology in the diagnosis.