This study was designed to establish whether 40 mg omeprazole once daily exhibits sufficient additional efficacy over that of 20 mg omeprazole once daily in patients with symptomatic reflux oesophagitis requiring more than an initial 4-week course of 20 mg omeprazole once daily (o.m.) to warrant routine use of the higher dose. Three hundred and thirteen patients were randomized to receive either 20 mg omeprazole (4 weeks) then 20 mg (second 4 weeks if not both healed and symptom-free after 4 weeks), or 20 mg omeprazole (4 weeks) then 40 mg omeprazole o.m. (second 4 weeks). One hundred and twenty-seven patients were healed and symptom-free after 4 weeks and left the study at that point. Taking the second treatment period in isolation, the healing rate (64% vs. 45%, P < 0.02) and relief of heartburn (72% vs. 60%, P < 0.002) were greater among patients receiving 40 mg omeprazole o.m., demonstrating the existence of a dose-response relationship for omeprazole. However, on completion, there were no significant differences between the patients randomized to the 20/20 mg (healed 65%, asymptomatic 69%) or the 20/40 mg (healed 74%, asymptomatic 74%: both not significant differences compared with 20/20 mg) regimens. The magnitude of the difference in efficacy between 20 and 40 mg omeprazole in symptomatic reflux oesophagitis is insufficient to warrant the routine use of 40 mg in patients requiring more than 4 weeks' treatment with 20 mg omeprazole o.m.; continued treatment with 20 mg omeprazole for 4-8 weeks is the preferred option.