Idiopathic retroperitoneal fibrosis (RPF) is a rare disease entity and its etiology is uncertain. We report two similar cases which showed an uncommon presentation of idiopathic RPF. A 66-year-old woman and an 80-year-old man presented with incidental findings of left renal pelvic mass-like lesions. Computed tomography revealed a soft tissue density mass replacing the left renal pelvis, which was suspicious for renal pelvic cancer, and the diagnosis of idiopathic RPF was surgically confirmed. To the best of our knowledge, a few cases of idiopathic RPF presenting with features of a localized unilateral renal pelvic mass mimicking renal pelvic cancer have been reported.