We report a patient known to have an enterovesical fistula who presented severe acute metabolic acidosis during an episode of urinary retention. The enterovesical fistula which had been intermittently symptomatic for 4 years, had developed after several intestinal surgical procedures and related intraperitoneal sepsis following resection of colon cancer 21 years previously. The patient who had a total colectomy and ileostomy, was admitted for hip replacement with the routine placement of a Foley bladder catheter. Three weeks post-operatively, the patient developed acute urinary retention following removal of the urinary catheter. The output from his ileostomy was immediately markedly increased, presumably from bladder urine diverted into the intestines through the enterovesical fistula. Within a few days he presented a normal anion gap metabolic acidosis with raised urea and stable creatinine; his clinical status deteriorated markedly with profound obtundation. These metabolic abnormalities were readily corrected by re-insertion of the Foley catheter with restoration of normal urine flow and immediate corresponding fall in the ileostomy output. Radiographic studies showed the presence of the enterovesical fistula originating from the jejunum. This is the first report of acute metabolic acidosis in association with an enterovesical fistula; the severe metabolic disturbances were triggered by the development of urinary retention resulting in the diversion of urine into the small bowel through the fistula.