A 72-year-old woman with poorly controlled hyperlipidemia and diabetes presented to the emergency department with a 5-day history of lower abdominal pain. She also had fever and reported nausea and vomiting. Physical examination revealed lower abdominal tenderness. Blood tests revealed leukocytosis associated with a left shift (neutrophil count of 11,800 per cubic millimeter) and elevation of the levels of C-reactive protein (24.0 mg per deciliter) and glucose (735 mg per deciliter [41 mmol per liter]). A plain radiograph of the kidneys, ureters, and bladder showed air surrounding the bladder (Panel A, arrows). An abdominal computed tomographic scan revealed an area of gas dissecting the bladder wall, bilateral hydronephrosis, and intramural gas with a cobblestone or beaded-necklace appearance (Panel B, arrows), findings consistent with emphysematous cystitis. The patient was treated with broad-spectrum antimicrobial agents and placement of a Foley catheter. Subsequently, a urine culture was positive for Escherichia coli; the patient was treated with antibiotics and recovered uneventfully. Emphysematous cystitis is a urinary tract infection that is associated with gas formation and is commonly caused by E. coli and Klebsiella pneumoniae.