This study was an open evaluation of omeprazole, 20 mg or 40 mg once daily, for the treatment of gastric ulcer. Thirty-four Filipino patients, 22 with gastric (GU) and 12 with prepyloric (PPU) ulcers at least 5 mm in diameter were recruited. In terms of ulcer size and number of ulcers per patient, the 40 mg group had more severe ulcer disease. Twenty-two patients (15 GU and seven PPU) received 20 mg omeprazole and 12 patients (eight GU and four PPU) received 40 mg for 2-8 weeks, according to healing. Ulcers were assessed endoscopically every 2 weeks and a biopsy was performed (when unhealed) to exclude malignancy. Symptoms were recorded at each visit to the clinic and also in daily diary cards. Ulcer-healing rates were assessed for all patients who fulfilled protocol requirements at any one visit and were as follows at 2, 4, 6 and 8 weeks, respectively, for 20 mg: 50%, 70%, 85% and 95%; for 40 mg: 50%, 75%, 92% and 92%; and for all patients: 50%, 72%, 88%, and 94%. Results at 4, 6 and 8 weeks excluded two patients who were lost to follow-up. All patients who completed the study and whose ulcers were less than or equal to 20 mm in size (24) were healed as well as six of eight patients with ulcers greater than 20 mm. At entry, all but one patient reported symptoms. After only one dose of omeprazole, 50% of the patients on each dose no longer reported pain in their diary cards and after 4 weeks most patients were symptom-free.(ABSTRACT TRUNCATED AT 250 WORDS)