Background and objectives: Dengue is a mosquito-borne disease affecting mainly tropical and subtropical regions of the world. The early diagnosis of dengue is required for identifying an epidemic and also for implementing effective vector control measures. This study was aimed to look at the utility of dengue NS1 (non structural protein 1) Ag as an early marker of dengue infection. Methods: The study was conducted on 200 clinically suspected cases of dengue, from the out- patients and inpatients attending various clinical departments of in and around Guntur. Patients presenting within the first week of fever were included in the study. Patients’ serum sample was subjected to ELISA for NS1 Ag, IgM Ab and IgG Ab. Results: Of the study group, 36.33% and 44.33% of cases were positive by NS1 Ag test and IgM Ab test respectively; when both the tests were combined together, the detection rate increased to 62.33%, which was statistically significant. In the first 3 days of fever, a higher number of cases were positive for NS1 Ag only, with a peak positivity of 42% on day 3. By day 4, a combination of NS1 Ag and IgM Ab gave a positivity of 28.22%. Day 5 onwards, a higher number of cases were positive for IgM Ab only, with a peak positivity of 33% on day 5. Conclusion: NS1 Ag is a very useful tool in the diagnosis of dengue infection especially in the first 3 days of fever. When used in combination with IgM Ab assay, the diagnostic algorithm significantly improves on a single serum sample. NS1Ag is an early diagnostic marker that is feasible in a routine diagnostic laboratory.