Purpose: This study was designed to investigate potential discrepancies between the depth and width of the mandibular denture anterior lingual flange and the alveolingual sulcus and to measure the changes in retention when flange lengths were modified to optimal dimensions in a sample population of elderly edentulous subjects. Materials and Methods: The anterior lingual flange and alveolingual sulcus were measured in 66 randomly selected elderly Taiwanese denture patients. The length and width of corresponding regions of the anterior lingual flange and alveolingual sulcus were compared. Mandibular denture retention was recorded and compared prior to and following modification of the anterior lingual flange with border molding. Results: A significant statistical difference existed between the mean anatomical dimensions of the alveolingual sulcus and the corresponding denture flanges. Following functional border molding, the mean retention of the mandibular denture increased in a statistically significant manner. Conclusion: The discrepancies between the anterior lingual denture flange and alveolingual sulcus dimensions appeared to significantly diminish mandibular denture retention and reinforce the value of proper flange extensions in the sublingual region.