PATIENT CHARACTERISTICS: Age: 18 and over Performance status: WHO 0-2 Life expectancy: Not specified Hematopoietic: Not specified Hepatic: Not specified Renal: Not specified Other: Not pregnant or lactating Fertile patients must use effective contraception No prior or concurrent malignancies unless: At least 1 year remission Low risk or recurrence No prior gastric malignancy No nonmalignant disease causing poor medical risk No allergy to omeprazole