This study aimed to investigate the occurrence of subpontic tissue enlargement (STE) beneath a mandibular fixed partial denture. A 55-year-old Japanese woman received periodontal therapy and cross-arch fixed partial dentures were placed in the maxilla and mandible. After 18 years, STE developed in the left posterior region of the mandible. It was presumed that biomechanical loading in the mandible, along with other factors, might have caused the STE in this particular patient.