A 67-year-old woman diagnosed with pyonephrosis and perinephric abscess because of an impacted urinary stone in the pelvicoureteric junction was admitted to the hospital with a high-grade fever. Although construction of a right nephrostomy for drainage of the abscess improved her general condition, she had a fever again 2 weeks after the initial treatment. Computed tomography revealed a persistent perinephric retroperitoneal abscess and a second drainage procedure was performed. Then, imaging examination revealed fistula formation between the cavity of the perinephric retroperitoneal abscess and the duodenum. The patient received conservative management including percutaneous drainage, discontinuation of oral intake, and antimicrobial chemotherapy. Three days after the second drainage and discontinuation of oral intake, imaging examination revealed complete closure of the fistula. Fistula formation between a perinephric abscess and the duodenum is very rare but a favorable outcome was obtained by our conservative management.