A pharmaco-epidemiologic study in hospitalized patients was carried out in order to establish the place and use of omeprazole (Losec), a new and promising drug in the treatment of acid-related diseases. A comparison is made with cimetidine and ranitidine use. It appeared that prescribed omeprazole doses were high in relation to the established defined daily doses and that substitution of H2 receptor antagonists by omeprazole led to tremendously increased drug costs. From the clinical indication data it could be established that the drug was prescribed appropriately.