Background. Occlusal rims are used to determine the jaw relationships in the transverse and vertical dimensions and estimate the inter-occlusal distance in edentulous patients. It is important to find ways to determine the height and shape of the occlusal rims correctly. This study was undertaken to determine the exact distance from the oral cavity floor to the incisal edges of mandibular incisors to serve as a guide for adjusting the height of the mandibular occlusal rim. Methods. Forty patients were selected and special trays were fabricated to prepare accurate stone casts on which the measurements were made at 0.01-mm accuracy. Two marks were placed on the casts at the incisal edges of mandibular incisors and at anterior attachment of lingual frenum. Then the distance between these two marks was determined on the vertical spindle of a surveyor using a digital Vernier measuring tool and recorded. Results. The results showed that the mean and standard deviation of the distances between the oral cavity floor and the incisal edges of lower incisors were 14.35 ± 1.68 mm, with a range of 10.2‒17.02 mm. The mean distances in males and females were 15.42 ± 0.97 and 13.28 ± 1.57 mm, respectively. T-test showed significant differences in this distance between males and females, with greater distances in males. Conclusion. The distance between the oral cavity floor and the incisal edges of mandibular incisors at anterior attachment of lingual frenum might be a proper criterion for the initial adjustment of occlusal rims.