The alveolar bone loss in 24 patients with chromosomally verified Down's syndrome, and in a control group of 28 mental retardates from the same institution, was examined using orthopantomography. The same patients had been examined 5 years earlier, and the present results were compared with the former. During this 5-year period the patients did not receive any periodontal treatment, only daily brushing of the teeth. A clear difference between the two groups was evident at both times: the percentage of affected teeth in the study group was 25% in 1975 and 47% in 1980 as against corresponding figures of 1.8% and 6.8% in the control group. The prevalence rate of bone loss of 5 mm or more showed little or no increase in the patients with Down's syndrome (69% to 75%) whereas in the controls the rate of increase was greater (20% to 43%). In individual teeth, i.e. the mandibular first molars, progress was also more rapid in the controls, 28.3%, as compared to 12.7% in the study group.