The purposes of this study were to determine the association, in 10 pedigrees, between adenomatous polyposis coli, hereditary nonpolyposis colorectal cancer, and occult radiopaque jaw lesions, and to assess whether these radiodensities are predictors for adenomatous polyposis. In seven kindreds with adenomatous polyposis, all patients with polyps had jaw lesions; in one kindred, no jaw lesions were found. In one of two kindreds with hereditary nonpolyposis colorectal cancer, no affected individuals had jaw lesions. In the other, the 1 affected patient with dental radiographs had generalized jaw lesions. Twelve children less than 16 yr old at risk for adenomatous polyposis were observed. Seven children with jaw lesions developed polyps after a mean interval of 4 yr. Five children without jaw lesions were polyp-free during a 5-10-yr follow-up. Thus, occult jaw lesions are consistently found only in some families with adenomatous polyposis coli, providing support for heterogeneity in polyposis syndromes. Jaw lesions are good predictors for polyp development in kindreds with adenomatous polyposis coli and jaw lesions. Their role as markers in hereditary nonpolyposis colorectal cancer needs exploration.