Context: Brain is the most majestic and intricate organ in the human body. The ventricular system consists of a series of interconnecting spaces and channels within the brain which are derived from the central lumen of the neural tube. Third ventricle is the cavity of diencephalon that lies between the two thalami and is related to the most important structures of brain. Abnormality arising from any of these structures can obstruct the third ventricle affecting the cerebrospinal fluid drainage. It is important to distinguish the pathological ventricular enlargement from age related ventricular enlargement. Aims: To estimate the transverse diameter of third ventricle with respect to age using magnetic resonance and imaging in adults of Kashmiri population. Settings and Design: Observational study Methods and Material: The axial MR images of 100 patients were reviewed in the Department of Radiodiagnosis, GMC, Srinagar. The transverse diameter of third ventricle was taken at the level of interventricular foramen between the two thalami. Statistical analysis used: SPSS Results: The transverse diameter of third ventricle ranged from 2.1mm -8.29mm with a mean of 4.17mm. The results revealed a significant increase in the transverse diameter with increasing age. Conclusion: Knowledge about the mean width of third ventricle in different age groups and in either gender may help to distinguish the pathological ventricular enlargement from age related ventricular enlargement.