The ileostomy output and electrolyte loss was studied in 23 patients undergoing operation for ulcerative colitis, to determine corticosteroid influence. Ten patients did not use corticosteroids and 13 patients received postoperatively 400 mg hydrocortisone intravenously for 4 to 5 days followed by 90 mg prednisone per os thereafter. Ileostomy volume and electrolyte concentration and total loss were measured on the 4th and 8th postoperative days. Patients on corticosteroids presented with a reduction of volume lost from the ileostomy (P less than 0.001), lower sodium concentration and total loss (P less than 0.00001), higher potassium concentration (P less than 0.00001) with similar total loss (P is not significant) and higher calcium concentration (P less than 0.00001) but lower total loss (P less than 0.05). It is concluded that corticosteroid administration influences significantly both volume and electrolyte loss due to the ileostomy discharge in patients operated for ulcerative colitis, suggesting a need for careful maintenance of fluid and electrolyte balance in the early postoperative period.