Aim- 1. The aim of this study was to investigate the average maximum range of inter-incisal mouth opening in a representative sample of the adult subjects of Haryana. 2. To see any correlation between maximal inter-incisal opening with age. Methods- Maximum mouth opening was studied in 756 adult subjects with age range of 20-50 years in Yamunanagar, Haryana. Age limit was further divided into three groups (20-30, 31-40, 41-50). Those with clinical history of TMJ involvement, OSMF, any trauma, odontogenic and non-odontogenic infections, dental prosthesis on the anterior teeth, congenital anomalies in the maxillofacial region were excluded from this study. The measurements were recorded twice and mean of the two values were taken. Statistical Analysis- Independent sample t-test was calculated to compare age and mouth opening in both male and females respectively. Bivariate pearson correlation was used to see any relationship between age and mouth opening. P-value ≤ 0.05 and CI (confidence interval) at 95% were considered statistically significant. The Results- The average mouth opening of males (45.36±6.70 mm) subjects was higher as compared to female (41.27 ± 6.75 mm) with significant, p-value 0.000. The mean mouth opening ± SD for both sexes combined was 43.39 ± 7.02 mm. The corresponding values for mean inter-incisal opening in male population aged 20-30, 31-40, 41-50 were 45.52 ± 7.15, 46.16 ± 5.47, 42.96 ± 6.82 mm and in female population aged 20-30, 31-40, 41-50 were 41.40 ± 7.08, 41.60 ± 6.29 and 40.03 ± 6.38 mm respectively. Conclusion- Maximal mouth opening differ significantly with gender. There is a decrease in MMO with older age group.