Objective:To investigate the outbreak of unknown fever at Siliguri town,Darjeeling District on request from the State Health Department,Government of West Bengal.Methods: Investigations were made to the affected wards,Sub Divisional Hospital and the nursing homes of Siliguri Town.Duration of illness was 3-5 days.Interesting observations were made in some cases which had gastrointestinal disorders with high serum glutamate pyruvate transaminase(SGPT) and serum glutamic oxaloacetic transaminase(SCOT) levels.A total of 69 blood samples and 7 throat swabs(in Minimum Essential Media) were collected and brought to the ICMR Virus Unit, Kolkata for analysis.Mosquitoes from different affected areas were collected for the identification of the definite vector.Results:Amongst the 69 blood samples,42(60.86%) were positive to IgM antibody against dengue virus by Mac enzyme-linked immunosorbent assay(ELISA) test.No IgM antibody to Japanese encephalitis virus was detected among the collected blood samples.Based on the clinical symptoms,presence of IgM antibody to dengue virus and identification of Aedes mosquito,it amply proves that,the illness of those cases were due to dengue virus infection. Conclusions:Based on clinical-epidemiological observations of the investigations the possibility of a communicable disease of viral origin,the detection of IgM antibody and the identification of Aedes egypti,and the potential circulation of denge virus in Siliguri town for the first time were all suggested. Objective: To investigate the outbreak of unknown fever at Siliguri town, Darjeeling District on request from the State Health Department, Government of West Bengal. Methods: Investigations were made to the affected wards, Sub Divisional Hospital and the nursing homes of Siliguri Town. Duration of illness was 3-5 days.Interesting observations were made in some cases which had gastrointestinal disorders with high serum glutamate pyruvate transaminase (SGPT) and serum glutamic oxaloacetic transaminase (SCOT) levels. A total of 69 blood samples and 7 throat swabs (in Minimum Essential Media) were collected and brought to the ICMR Virus Unit, Kolkata for analysis. Mosquitoes from different affected areas were collected for the identification of the definite vector. Results: Amongst the 69 blood samples, 42 (60.86%) were positive to IgM antibody against dengue virus by Mac enzyme-linked immunosorbent assay (ELISA) test. No IgM antibody to Japanese encephalitis virus was detected among the collected blood sa mples.Based on the clinical symptoms, presence of IgM antibody to dengue virus and identification of Aedes mosquito, it a proof proves that, the illness of those cases were due to dengue virus infection. Conclusions: Based on clinical-epidemiological observations of the investigations the possibility of a communicable disease of viral origin, the detection of IgM antibody and the identification of Aedes egypti, and the potential circulation of denge virus in Siliguri town for the first time were all suggested.