Background/Aims: The effects of nocturnal gastric acid breakthrough(NAB) on Helicobacter pylori(H. pylori) eradication are yet unknown in peptic ulcer patients. The purposes of this study were to compare the effect of lansoprazole 30 mg b.d. to lansoprazole 60 mg o.d. on prevalence of NAB, and to determine whether NAB affects the eradication of H. pylori in peptic ulcer patients. Methods: Experiments were carried out in 67 patients having H. pylori positive peptic ulcers. They were randomized into two groups, one treated with a combination of lansoprazole 60 mg, clarithromycin 1.0 g, and amoxycillin 2.0 g once a day before breakfast(o.d. group), and the other, divided doses of the drugs were given before breakfast and dinner(b.d. group) for two weeks. Results: NAB occurred in 31 patients, 55.2% in o.d. group, and 39.5% in b.d. group(P=0.226). H. pylori eradication was achieved in 61.3% in NAB positive group and 83.3% in NAB negative group (P=0.055). The mean duration of NAB for H. pylori eradication group was 99.3⁑22.7 minutes, and 293.2⁑49.8 minutes for H. pylori persistence group(P＜0.05). The median intragastric pH of the H. pylori eradication and persistence group was 5.7⁑0.2 and 4.2⁑0.4, respectively(P＜0.05). Conclusions: Both the morning dose and the divided dose regimen of lansoprazole did not affect the intragastric acidity and occurrence of the NAB. NAB did not influence H. pylori eradication in peptic ulcer patients, but the duration of NAB and total intragastric median pH were found to influence the H. pylori eradication. (Korean J Helicobacter Res Prac 2002;2:178-182)