Contradictions exist in the literature regarding the effect of gastric secretion inhibition on phosphate absorption. In healthy controls, omeprazole would decrease the hyperphosphatemia or the hyperphosphaturia induced by an acute phosphate load, suggesting an inhibition of phosphate absorption. In chronic hemodialysis patients, gastric hypersecretion is associated with hyperphosphatemia, but inhibition of gastric hypersecretion by ranitidine in those receiving calcium carbonate (CaCO3) as a phosphate binder would paradoxically exacerbate their hyperphosphatemia. Because of these conflicting observations, we performed an open crossover study on 16 chronic stable hemodialyzed patients with a daily mean intake of 9.4+/-4 g of CaCO3, and we compared the plasmatic predialysis levels of phosphate, calcium, protides, bicarbonates, intact parathyroid hormone (PTH), urea, and creatininemia during 2 successive periods of 2 months, the first one without omeprazole and the second one with 20 mg omeprazole intake in the morning. Phosphatemia increased with omeprazole but not significantly from 1.80+/-0.38 to 1.89+/-0.42 mM whereas corrected calcemia decreased significantly (p = 0.04) from 2.41+/-0.18 to 2.36+/-0.16 mM as did bicarbonatemia from 26.7+/-3.5 to 25.7+/-3.1 mM (p < 0.05). No change in creatininemia or in blood urea was observed, suggesting the stable efficiency of dialysis as well as the stable intakes of protein and therefore of phosphate during the two study periods. In conclusion, inhibition of gastric secretion by omeprazole increases the plasmatic phosphate predialytic level but in a nonsignificant way. This increase may be explained by a slight but significant concomitant decrease of calcemia and bicarbonatemia. These results do not support the phosphate binding efficiency of CaCO3 being decreased by the inhibition of gastric acid secretion.