A 25-year-old man with suspected Gardner syndrome was introduced to our hospital by a dentist who, during examination of the patient, had found dental dysplasias and multiple osteomas of the jaw. Radiographs, endoscopy and biopsies revealed adenomatous polyposis of the colon. Genetic analysis of peripheral lymphocytes revealed a one-base deletion at codon 1556 in exon 15 of APC, which caused a frame shift and a premature stop at codon 1564. The pedigree analysis demonstrated five patients in his family who presented with dental abnormality and osteomas in addition to adenomatous polyposis of the colon. Although the relationship between the location of APC mutations and dental abnormalities remains controversial, this case supports the hypothesis that a mutation at around codon 1556 of APC is closely associated with dental abnormality and osteomas.