We present the case of a 49-year-old woman with poorly controlled diabetes mellitus of ten years' duration. She presented to the emergency department with nonspecific left lower abdominal pain. Ultrasonography showed mild left renal hydronephrosis, but the cause was not demonstrated. Computed tomography (CT) showed a swollen left kidney with a mildly dilated collecting system containing gas and dense fluid, confirming the diagnosis of emphysematous pyelitis. Management consisted of emergency antegrade nephrostomy drainage and parenteral antibiotics. Subsequently, the patient made a good recovery and was well when discharged home after two weeks. Gas-forming infections of the genitourinary tract carry high mortality and morbidity, and usually occur in patients with poorly controlled diabetes mellitus. Escherichia coli is the most common microorganism. This case emphasises the importance of CT in making an early diagnosis of emphysematous pyelitis, which allows prompt treatment and improves prognosis.