Hyperammoniemic encephalopathy has been reported after ureterosigmoidostomy. Its development is related to a problem of bacterial overgrowth and, most often, is favored by the presence of an underlying liver dysfunction. We report the case of a 43-year-old woman with a ureterosigmoidostomy done 28 years earlier who developed hyperammoniemic coma induced by an acute rectocolitis and in the absence of any detectable liver dysfunction. Neither administration of Lactilol and neomycin nor rectal tube drainage were effective; systemic antimicrobial therapy effective against the urease-producing gram-negative bacilli was required and led to a decrease in serum ammonia levels and a dramatic clinical improvement.