INTRODUCTION: Tuberculous meningitis (TBM) is the most dangerous form of extra pulmonary TB in many developing countries like India. It occurs in around 7– 12% of the entire TB cases3. Early diagnosis and treatment with antibiotic therapy and active management of the complications are of great importance to prevent the irreversible neurologic sequel and death. AIMS & OBJECTIVES: This study was carried out to determine the level of ADA activity in CSF of TBM, Pyogenic meningitis, viral meningitis and non infectious cases. To study the biochemical pattern and to correlate CSF protein and sugar concentration with CSF–ADA in TBM and Pyogenic meningitis, viral and non infectious cases. MATERIALS & METHODS: The present study was conducted over a period of 18months from Jan 2014 to June 2015 in the emergency laboratory of a tertiary care hospital in Guntur. A total of 580 CSF samples from all the medicine and the pediatric wards are included in the study. RESULTS: The critical cut–off value of ADA levels is calculated as >10 i.e.11 U/L with the help of ROC curve and  a sensitivity of  96.77 % and a specificity of 97.30 % is thus seen for the TBM group versus non TBM. As per the statistics, the association between the ADA levels and the lymphocytes, proteins with the ADA levels is also statistically significant (p < 0.0001) CONCLUSION: In conclusion, ADA level of 11 U/L and above in CSF could provide additional supportive evidence for the diagnosis of TBM in clinically suspected and bacteriologically negative cases.