Deep lingual undercut (LU) is commonly encountered in the posterior mandible, and is considered a risk factor in dental implants. In this study we investigated the value of data extracted from OPGs in predicting LU depth. Such predictors might be valuable in reducing the chance of lingual plate perforation (LPP) by recommending CBCTs prior to dental implant insertion when deep LU is anticipated We aimed at assessing the following variables as potential predictors of LU depth: 1) alveolar process height (measured on OPGs), 2) alveolar process width (measured on CBCTs), and 3) the distance from apical region of dental alveoli to superior margin of IAN canal (measured on OPGs). 128 CBCTs and corresponding OPGs of posterior mandibles of 128 patients (70 females, 58 males; age range=18-87 years, mean age=45.8 years, SD=17.0 years) were used. Only dentate sites of lower first (LM1) and second molars (LM2) were considered. Four predictors of LU depth were found, the strongest was the ratio between alveolar process width (which could be assessed clinically) and alveolar process height as measured on OPGs (r=.454 at LM1 site, r=.592 at LM2 site). Predictors derived from OPG measurements might be valuable in anticipating LU depth and might be more valuable when combined with alveolar process width (which might be assessed clinically). We recommend considering the suggested predictors in assessing the need of CBCT prior to immediate dental implant insertion in posterior mandible.