Omeprazole is a proton pump inhibitor which reduces both basal and stimulated gastric acid secretion by inhibiting the parietal cell enzyme H(+)-K(+)-adenosine triphosphatase. Typical adverse effects of proton pump inhibitors are nausea, diarrhea, constipation and endocrinologic abnormalities such as gynecomastia. Cutaneous side effects are rare but may include contact dermatitis, lichenoid eruption, leukocytoclastic vasculitis or toxic epidermal necrolysis; anaphylaxis is rare. We identified omeprazole as the causative agent by oral challenge test in a patient who had developed anaphylaxis while taking several drugs.