Lubiprostone is a new laxative, that locally activates the type-2 chloride channels in the gastrointestinal epithelium, inducing net fluid secretion. We examined the effects of lubiprostone on the serum levels of electrolytes. Twenty-four μg lubiprostone was added to conventional laxatives every day in 7 hemodialysis patients who had chronic constipation. The changes in defecation frequency and electrolyte concentrations induced by lubiprostone were examined after 2, 4, 6, and 8 weeks. Lubiprostone reduced defecation in all 7 patients, but 1 patient stopped taking it after 2 weeks because they suffered nausea, and 2 patients stopped taking it after 4 and 6 weeks, respectively, because of diarrhea. The mean serum phosphate level had decreased from 5.8±0.9 to 5.1±0.8 (p=0.034, n=7) after 2 weeks’ lubiprostone treatment. After 4 weeks, 5 of 6 patients exhibited reduced serum phosphate levels, and after 8 weeks all 4 of the remaining patients continued to exhibit reduced serum phosphate levels. The serum phosphate levels of all 3 patients who stopped taking lubiprostone rose again. No changes in predialysis systolic blood pressure were observed, but the volume of water removed was decreased in 6 patients. One patient exhibited difficulties with water removal after 2 weeks. In conclusion, lubiprostone reduced hyperphosphatemia in hemodialysis patients with chronic constipation.