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Dengue fever is an Aedes aegypti mosquito-borne viral infectious disease found in tropical and subtropical areas. Thailand is one of the endemic areas in which seasonal dengue outbreaks usually occur. Laboratory diagnosis of dengue infection can be done by NS1 antigen detection in the patient’s serum at the early stage of disease (1-7 days after infection). Another method is to detect IgM anti-dengue antibodies, which usually rise 3-5 days after the onset of fever. The IgG anti-dengue antibodies are detected at the later stage of infection and usually stay positive for a long time, thereby indicating the previous dengue infection. This study aims to investigate the incidence of dengue virus infection in suspected dengue virus-infected patients, who visited Siriraj Hospital during January 2012 - December 2016. Here, we analyzed the test results of 5,196 patients’ serum samples that were processed for dengue NS1 antigen and/ or IgM antibody detection using immunochromatographic assay and found that 1,813 serum samples were positive for either dengue NS1 antigen or dengue specific IgM antibody or both. The result demonstrated that dengue virus infection rate was 36.38% (95% CI = 32.91, 39.99), 40.75% (95% CI = 37.71, 43.86), 34.64% (95% CI = 30.82, 38.67), 43.33% (95% CI = 40.93, 45.76) and 17.78% (95% CI = 15.69, 20.07) in 2012, 2013, 2014, 2015, and 2016, respectively. The fiveyear average incidence of dengue infection was 34.89% (33.60%, 36.21%). The highest numbers of dengue-infected cases were in 2015 (43.33%), while the lowest case numbers were in 2016 (17.78%). High rate of dengue virus infection was observed during November to December. Interestingly, dengue virus infection is the mostly detected in patients aged between 15-24 years. Infection rates between females and males were not significantly different (Female : Male = 1.2:1; p = 0.29).