Intermittent hyperammonemic encephalopathy after ureterosigmoidostomy is a rare, but if unrecognized, potentially lethal condition. Ureterosigmoidostomy was performed in a male patient with bladder extrophy. After 35 years, he developed hyperammonemic encephalopathy. Diagnostic procedures did not reveal hepatic nor metabolic disorders. Despite administration of preventive medical treatment, several episodes recurred. A durable prevention was finally achieved by conversion into an ileal conduit. Intermittent hyperammonemic encephalopathy can occur decades after ureterosigmoidostomy. In the case of absence of metabolic disorders and resistance to medical treatment, conversion into a urinary diversion using an ileal segment constitutes an effective ultima ratio.