Ten patients with an end jejunostomy and one with a jejuno-rectal anastomosis (jejunal length 30-140 cm) ate a constant chosen diet for 2 control days, and 2 test days when 40 mg omeprazole orally was taken each morning. In the 7 patients with a net secretory output of fluid, there was a mean reduction in wet weight of 0.66 kg/24 h (range -0.16 to 1.45 kg/24 h; P less than 0.05) and sodium 46 mmol/24 h (-51 to 135 mmol/24 h; N.S.); the four patients with net absorption of fluid showed no reduction in intestinal output. One patient with 30 cm jejunum responded little to oral but did so to intravenous 40 mg omeprazole twice a day with a reduction in wet weight of 3.00 kg/24 h and sodium 157 mmol/24 h. In one patient oral 40 mg omeprazole daily gave equivalent results to oral 300 mg ranitidine twice daily; in 2 others it was equivalent to intravenous 50 micrograms octreotide twice daily. Omeprazole reduces the intestinal output in patients with the short bowel syndrome and a net secretory output, but not enough to prevent the need for parenteral fluid and electrolyte replacement.