Dengue fever is the arboviral disease with the most significant impact in public health. In Colombia, the largest outbreak ever recorded occurred in 2010 with at least 151,774 cases. Understanding the factors and rates of transmission in schoolchildren are needed towards characterizing the burden of disease in the community and defining strategies for epidemic control. In this study, the incidence, seroprevalence and circulating serotypes of DENV were determined in schools from three different neighborhoods of Medellin (San Javier, Poblado and Laureles). A cohort containing 2,340 volunteer students from two public and one private school including primary and high schools (ages 5-19) was established. In the cross sectional study, blood samples were obtained from all admitted students and specific dengue IgM ELISA were performed. The longitudinal study involved surveillance of absenteeism of enrolled students due to febrile illness shorter than 7 days. Standardized physical exam were performed and venous blood samples were obtained from ill students during both acute and convalescent stages. Dengue diagnosis was confirmed using RT-PCR and IgM ELISA. Among the 2340 students enrolled, 53% were women and students of all grades were represented. In the cross sectional study, 69 (2.9%) students were positive for IgM antibodies. Their mean age was 11.4 years (range = 5 to 19 years) and the distribution of cases by sex was the same. In the longitudinal study, amongthe 146 students declared ill because of absenteeism due to febrile illness,12 (8.2%) had IgM antibodies against dengue and DENV-1 serotype was detected by RT-PCR in three of them. The highest frequency of DENV seropositive cases was detected in San Javier¿s school (3.9%), followed by the Laureles¿ school (3.4%). In this first phase of the study, a high incidence of dengue fever was found in school children mirroring the large outbreak experienced in 2010.