Extracolonic lesions compatible with Gardner's syndrome were studied in 34 patients from 17 unrelated families with established familial adenomatosis coli. Thirteen patients (38 per cent) had the complete syndrome and 12 patients (35 per cent) a bisymptomatic syndrome. Bone lesions were found in 79 per cent of the patients examined, epidermoid cysts in 35 per cent, desmoid tumours in 18 per cent and dental changes in 18 per cent. One extracolonic manifestation of the triad in at least one affected family member was detected in 14 families (82 per cent). The findings provide additional evidence for the view that all cases of familial adenomatosis coli belong to a single genetic entity. The usually innocent extracolonic signs of Gardner's syndrome severe as valuable markers for early detection of asymptomatic family members at risk. Panoramic X-ray of the jaws seems to be especially suitable for this purpose.