Purpose: The purpose of this study was to investigate the influence of mandibular residual ridge shape on masticatory efficiency in complete denture wearers. Materials and Methods: Twenty-eight complete denture wearers (mean age 75.6 years, SD 4.8) who received their complete denture treatment by one of the authors and had no troubles in daily use were selected. The mandibular residual ridge was evaluated by using a replica of the ridge obtained with heavy-body silicone impression material. The basal area of the replica was measured by a digitizer. The volume and the height of the replica were calculated. Masticatory efficiency in each subject was measured by the sieving method. Results: There was a significant correlation between the masticatory efficiency and basal area, volume, and height of the residual ridge. The basal area showed the strongest correlation. Conclusion: The basal area of the denture foundation greatly influenced the masticatory efficiency, suggesting that the masticatory efficiency in complete denture wearers was limited by their own residual ridges and that clinicians should inform their patients about the limitation of the recovery of masticatory ability before the beginning of denture treatment.