Although rarely used today for supravesical urinary diversion, ureterosigmoidostomy was commonly utilized in patients with bladder exstrophy. We report an unusual case of iliopsoas abscess developing 30 years after an ipsilateral nephrectomy in a patient with bladder exstrophy who had undergone ureterosigmoidostomy for urinary diversion more than 50 years prior. The etiology appeared to be persistent ureteral reflux and fistulization from a patent ureterosigmoidostomy stump. After percutaneous drainage of the abscess and intravenous antibiotic therapy, the patient was managed with complete excision of the ureterosigmoid anastomoses and creation of an ileal conduit urinary diversion.