Objective: The aim of the study was to determine the relationship between fermented and unfermented dairy product consumption and Helicobacter pylori seropositivity in a Mexican population. Design: Dietary interviews were conducted as part of a population-based case-control study in 2005. Serum was obtained for each participant to determine H. pylori seropositivity status. Adjusted odds ratios were estimated from multivariate logistic regression models. Setting: Mexico City, Mexico. Subjects: A random sample of 464 healthy adult residents. Results: The overall seroprevalence of H. pylori in the study sample was 75.4%. In fully adjusted models, compared with those who did not consume yoghurt, there was a protective effect of eating up to one serving per week of yoghurt and more than one serving per week of yoghurt (odds ratio (OR) = 0.57, 95% confidence interval (CI) 0.35-0.94 and OR = 0.45, 95% CI 0.24-0.86, respectively), with a P for trend of 0.01. There were no effects for the consumption of unfermented dairy products (milk and cheese). Conclusions: This study suggests that yoghurt consumption may have a protective effect against H. pylori seropositivity. Additional studies are needed to determine whether consumption of yoghurt or other fermented dairy products can prevent or eradicate H. pylori infection.