Water and electrolyte balance after ileoanal anastomosis. Water and electrolyte balance was studied in 30 patients with ileoanal anastomosis and J pouch, 10 patients with conventional ileostomy, and nine nonoperated patients with quiescent ulcerative colitis. Serum electrolyte concentrations, daily urinary volume, and daily losses of sodium, potassium, and chloride were measured in all patients. Daily fecal weight and daily losses of sodium and potassium were analyzed in patients with ileoanal anastomosis or conventional ileostomy. Serum chloride in patients with ileoanal anastomosis was significantly lower (P less than 0.05) than in those with conventional ileostomy or in nonoperated patients. Daily urinary loss of sodium in nonoperated patients was significantly higher than in patients with ileoanal anastomosis (P less than 0.01) or conventional ileostomy (P less than 0.05). Daily urinary loss of chloride in patients with ileoanal anastomosis was significantly lower (P less than 0.05) than in nonoperated patients. Daily fecal loss of potassium in patients with ileoanal anastomosis was significantly higher (P less than 0.05) than in those with conventional ileostomy. Daily urinary volume and fecal weight did not differ significantly in patients with ileoanal anastomosis or conventional ileostomy. The present study indicates that changes in water and sodium balance after ileoanal anastomosis are similar to those after conventional ileostomy but chloride balance is more altered after ileoanal anastomosis. 