Sodium and potassium in the ileostomy output and urine were determined in 28 patients with ulcerative colitis on a free diet and in eight patients on a defined constant diet, before and after conversion from a conventional ileostomy (CI) to a continent reservoir ileostomy (RI). Feces and urine were collected both in the hospital and at home. Patients with CI on free diet had a median intestinal loss of 62 mmol sodium and those with RI 74 mmol/24 h collected in the hospital (p < 0.05). The figures for at home was 79 and 81 mmol/24 respectively, and were larger than in the hospital (p < 0.01). Sodium loss in the urine (U-Na) and the intake of sodium did not change significantly after conversion. Patients with a low U-Na before conversion also had a low U-Na after, in a few almost nil, implying a need for increased intake of sodium. Patients with a CI and low urinary output of sodium should be carefully studied with respect to their sodium balance before accepting them for conversion to RI. The ileostomy output of potassium increased after conversion (4.3 vs. 6.8 mmol/24 h; p < 0.01) in the hospital (5.3 vs 7.1 mmol/24 h; p < 0.01) at home. Patients on a defined constant diet before and after conversion did not show any significant differences in absorption of sodium, potassium, magnesium or calcium after conversion, but did show a reduced dry weight of the ileostomy output, indicating an increased degradation of intestinal contents in RI patients.(ABSTRACT TRUNCATED AT 250 WORDS)