Omeprazole is the first drug designed to block the final step in the acid secretory process within the parietal cell. It has been shown to be extremely effective in the treatment of peptic ulcer disease, reflux esophagitis, and the Zollinger-Ellison syndrome. Although clinical experience with omeprazole is still limited, many controlled studies have established the short-term safety of this drug. We report the first case of a serious short-term adverse reaction with the use of omeprazole: hemolytic anemia. The patient developed weakness, lethargy, and shortness of breath 2 days after starting therapy with omeprazole. Two weeks after the initiation of therapy, her hematocrit had decreased from 44.1% to 20.4%, and she had a positive direct Coombs antiglobulin test and an elevated indirect bilirubin. After she discontinued the omeprazole, her hemoglobin and hematocrit gradually returned to normal. The mechanism by which omeprazole caused the patient's hemolytic anemia is uncertain, but physicians should be alerted to this possible adverse effect.