Aim: to analyse patterns and prevalence of different ABO blood groups in dengue and to find an association between ABO blood groups and severity of dengue. Material and methods: This prospective observational study was done the Upgraded Department of Paediatrics, Patna Medical College & Hospital, Patna, Bihar, India, for 1 year. Children between 0-18 years of age were included. All patients with serological confirmation of Dengue (NS1, IgM/IgG positivity) by Rapid Card methods with hematology data were included. Blood groups (by Forward Blood grouping -Slide method with Anti-A, Anti-B sera from Tulip diagnostics) were also recorded. The severity of dengue was graded according to World Health Organization guidelines. Results: Of the 400 cases, 230 were blood group O, 65 were blood group A, 88 were blood group B and 17 were blood group AB. The control group for blood group distribution was determined by assessment of blood groups of 200 random patient’s blood sample. Of the 214 patients who presented with dengue fever without warning signs, 134 (62.62 %), 38(17.76%), 36 (16.82%) and 6 (2.81%) had blood groups O, A, B, AB respectively. When compared with general population, this was statistically significant (ϰ2=55.65, degrees of freedom=3, p=9.19x10-11). Blood group O presented with milder forms of dengue as compared to distribution in general population (39%). Of the 70 patients with dengue fever with warning signs, 33 (47.14%), 11 (15.71%), 22 (31.43%) and 4 (5.71%) had blood groups O, A, B, AB respectively, which was statistically significant (ϰ2=31.47, degrees of freedom=3, p=1.19x10- 6 ). Conclusion: Dengue infection is prevalent in most parts of India with severe forms having high mortality. This study concludes that although the incidence of dengue fever is higher in children with blood group O, AB blood group is associated with severe forms of dengue, especially in secondary infections.