In the absence of an effective vaccine, vector control and surveillance of dengue fever (DF) and dengue hemorrhagic fever (DHF) are the most important strategies currently used to reduce the impact of these diseases in affected population. The objectives of this study were to estimate the incidence of symptomatic and asymptomatic dengue cases, the prevalence of antidengue antibodies, and to evaluate the laboratory and clinical aspects related to an active surveillance of dengue cases. In this study, active surveillance was incorporated as a part of the study design. At total of 3,255 people from four high risk neighborhoods were followed in a two years prospective study whereby the participants' houses were visited three times a week. During these visits, dengue cases were characterized by identifying patients with fever as well as other symptoms that were compatible with dengue disease. In addition, a biannual blood sample was taken for each study participants, to establish the prevalence and six month incidence of dengue infection. We found a crude incidence density (ID) of 3.24 by 100,000 person/days (p/d) which changed from 5.69 by 100,000 p/d for the first year of the study to 1.45 by 100.000 p/d in the second year. In both years, the months from July through September had the highest ID of 8.81 by 100,000 p/d. Children displayed higher ID when compared to adults, RR: 3.92 (2.38 - 6.48). The Plaque Reduction Neutralization Test was used to assay for the presence of antidengue antibody in 2,125 study participants (65.3% of total). The prevalence of anti dengue antibodies was found to be 86.6% (1,840 positives). The prevalence of anti DENV-1 was 74%, while 65.2 % of the participants had anti- DENV-1 and anti- DENV-2 simultaneously. The cumulative incidence of anti IgG dengue antibody in the negative participants (283 at the start of the study) was 30% in the first 6 months period, 29.6% in the second 6 months, and 23.8 in the third one. The difference between the numbers of participants detected in the active surveillance, (270 confirmed and non confirmed dengue cases) with the numbers of people who showed sero-conversion to anti-IgG dengue antibody within a relatively short period of time suggested that there was a high number of asymptomatic dengue infections present in the population. Transmissibility of the virus, the surveillance of dengue, and vaccine implementation in the near future would all be affected by the large number of asymptomatic people in hyperendemic countries.