Background and Aims. Intrafamilial tramission is considered as a major route of Helicobacter pylori infection. This cross-sectional study was design to evaluate the factors associated with intrafamilial H. pylori transmission and seroprevalence of H. pylori infection among Thai families. Patients and Methods. Clinical information, structured questionnaires, and blood samples were obtained between June 2002 and May 2003. The structured questionnaire was completed for all family members by the same researcher. H. pylori status assessed by anti-H. pylori serology (HM-CAP; Enteric Product Inc., Westbury, NY and HB 2.1; Genelabs Diagnostics, Singapore). Results. Total of 140 family members including 71 H. pyloripositive family members and 69 H. pylori-negative family members were evaluated in this study. The seroprevalence of H. pylori infection in H. pylori-positive family members was significantly higher than family members of H. pylori-negative (74.7% versus 39.1%; p < .001). The multiple logistic analysis demonstrated that age (OR = 6.3; 95%CI = 2.1–19.2, p < .01), share drinking cup behavior (OR = 10.5; 95%CI = 2.7–40.5, p < .001), and belong to family member of H. pylori-positive patents (OR = 10.8; 95%CI = 4.1–28.5, p < .001) are the independent risk factors of H. pylori infection. Conclusion. This study supported the role of intrafamilial transmission of H. pylori infection among Thai families. Improvement of personal and family hygiene should be emphasized for prevention of H. pylori infection.