Neonatal Hyperbilirubinemia is a common condition in neonates and the most common pathological cause leading to hyperbilirubinemia is Rh or ABO incompatibility. Although most newborns with jaundice are otherwise healthy, the greatest risk associated with indirect hyperbilirubinemia is the development of bilirubin induced neurologic dysfunction which typically occurs with higher indirect bilirubin levels. Aim of the Study: This study was done to (1) Find out the incidence of early hyperbilirubinemia and need for phototherapy in babies born to O+ve mother and (2) Cord Bilirubin as a marker for early phototherapy in ABO blood group incompatibility. Method : This was a prospective cohort study which included 50 consecutive term appropriate for gestational age babies with blood group either A or B born to O+ve mother as cases and another 50 consecutive term appropriate for gestationalage babies with O+ve blood group born to O+ve mother served as control. Result: 88% of the babies in the case group required phototherapy for > 24 hours whereas in the control group only 55% required phototherapy for > 24 hrs. Based on statistical analysis, there was a probability that the newborns who needed early phototherapy had a cord bilirubin 2.05 or more. Conclusion: It may be concluded that babies with ABO incompatibility needed early and prolonged treatment compared to controls. A cord bilirubin above 2.05 can predict the need for treatment.