Background: Blood is a significant fluid of the human body that is responsible for the flow of essential nutrients, hormones, enzymes, as well as oxygen in our body. Blood groups are essential for blood transfusions, organ transplantation. Blood groups antigens can be used in forensic pathology, genetic research and anthropology. Objective: The objective of this study was to estimate the frequency of ABO blood groups in first year MBBS students of CMH Medical College, Lahore and to find correlation between blood groups and BMI. Study design:  A cross-sectional study Place &duration:  CMH Lahore Medical and Dental College, Physiology Department. 2 months. Methodology: The study was carried out on undergraduate first year MBBS medical students of CMH Lahore Medical College. After taking consent and filling the proforma ,height and weight were measured using standard method thus BMI was calculated  . Blood groups were determined by ABO blood group were determined in a practical in Physiology laboratory by conventional glass slide method. Results: Blood group B Positive was most predominant blood group (41.2%),No relationship between blood groups and BMI was observed. Conclusion: Our finding relate that Blood group B is most prevalent in the first year MBBS students of CMH LMDC. There is no relationship between blood groups and BMI.