Background: It is known that blood group antigens are related to the development of peptic ulcer and gastric carcinoma. Infections due to H. pylori are most widespread among the developing regions due to poor standard of public health. This study sought to determine the association of H. pylori with ABO blood groups, age, gender, and smoking status among inpatients at a public sector hospital in Karachi. Materials and Methods: A cross-sectional study was conducted at endoscopy suite at a public sector hospital in Karachi in the year 2011. All the symptomatic patients coming for upper GIT endoscopy were included in this study. Results: Biopsy for histopathology was taken from 93 patients, with an age range from 15-65 years. Age group of 15 to 20 years was found to be associated with H. pylori infection but without significance (p-value 0.83). In all, 36 (38.7%) turned out to be H. pylori positive with a significant male preponderance (p=0.04). Distribution of ABO blood groups in H. pylori positive group were A=31.4%, B=15.4%, AB=25.0% and O=53.7%, with a statistically significant link for blood group O (p=0.05). Rhesus factor was also compared but significant relationship was evident (p-value 0.73). Conclusions: This study demonstrated that H. pylori infection can be related to ABO blood group, middle age persons and male gender. People of blood group O are more prone to develop infection related gastritis, ulcers, and even perforations, so they should be more cautious against transmission of the bacterium.