Although hyperammonia caused by hepatic failure is a well-known cause of coma, it is less well described in the absence of detectable liver dysfunction. We describe a case of a 54 year old woman with a decreased level of consciousness in the presence of hyperammonaemia. Laboratory studies showed normal coagulation and liver enzyme tests. In retrospect she suffered from memory- and concentration problems, somnolence, frequent vomiting and headaches. She underwent an ureterosigmoidostomy in childhood for a congenital bladder abnormality. With ureterosigmoidostomy large amounts of urea are in contact with the large bowel and its bacterial contents. Ammonia can be produced by urea-splitting bacteria, which can be present in the diversion. Due to bacterial overgrowth, especially with urease producing gram negative bacilli, hyperammonaemia can occur. We provide an overview of non hepatic causes of hyperammonemic coma and discuss therapeutic options. Initially our patient received a common treatment including protein restriction and lactulose. She also received sodiumbenzoate and carnitine for a short period of time. After being discharged from the ICU she suffered from relapses, which only ended when her ureterosigmoidostomy was converted into a Bricker-type ureteroileostomy. © 2011, Nederlandse Vereniging voor Intensive Care.