The prevalence rates of periodontitis and dental caries in 30 Down's syndrome patients and 30 matched, otherwise retarded, controls were compared. The populations were older than those usually studied, with mean (+/- SE) ages of 27.4 (+/- 2.1) and 28.9 (+/- 2.3) for the Down's syndrome and control groups, respectively. Bone loss was found in 60.0% of sites in the Down's syndrome patients, compared with 9.3% sites in controls. In Down's syndrome, bone loss was first seen at about age 16, with 92% of patients 16 or older having loss; in contrast, only 42% of control patients 16 or older had bone loss, which first appeared at about age 28. There was a considerable difference in caries prevalence in the two groups, with the Down's syndrome patients having a lower overall prevalence which was most noticeable with respect to interproximal lesions. Seven Down's syndrome patients over age 15 were caries-free, as opposed to only one control patient.