OBJECTIVE: To report a case of hyperammonemic encephalopathy in the absence of liver disease caused by a urinary tract infection with a previously unreported organism. DESIGN: Case report. SETTING: Pediatric intensive care unit. Patient: Critically ill child who presented with hyperammonemic coma. INTERVENTION: Bladder drainage and antimicrobial therapy for the urinary tract infection. Measurements: Rapid resolution of the hyperammonemia. MAIN RESULTS: Rapid improvement in the patient's neurologic status and a return to his baseline function without neurologic deficit. CONCLUSIONS: Patients with abnormal urinary collecting systems who present with altered mental status should have a serum ammonia concentration sent as part of the initial evaluation. We have added Providencia species to the list of documented causative organisms in this rare association between urinary tract infections and hyperammonemia.