A comparative study of the vertical forces of mastication was made in patients with conventional (soft-tissue-borne) dentures and in patients with mandibular subperiosteal implant dentures. Twelve patients served as subjects for this study. Two of them were studied before and after the insertion of a mandibular subperiosteal implant. The vertical forces of mastication were measured in pounds at various degrees of vertical opening. The results showed that the patients wearing mandibular subperiosteal implants were able to exert greater forces than those using conventional dentures. © 1969 The C. V. Mosby Company.