AbstractCharacteristic radio‐opaque lesions were detected in the mandibular bone, most clearly on the panoramic X‐ray, in 27 or 93·2 per cent out of 29 cases in 15 families with familial polyposis coli, including 21 cases without Gardner's stigmata, 7 cases with the incomplete syndrome and 1 with the complete syndrome.Only 3 out of 19 members of these families who were free from polyposis had radio‐opaque jaw lesions. The cases with polyposis of other types also showed a negative result.The radiological diagnosis suggested that the lesions were of the character of endosteoma. Our observation is considered to be significant evidence in the understanding of the pathogenesis of Gardner's syndrome as well as a useful method for the early detection of carriers in the polyposis