It has been suggested that periodontal diseases have declined during the last 2-3 decades in many populations. However, the present observations on marginal bone loss among 326 Norwegian dental patients aged 20-70 years, examined 35 years ago, were similar to comparable data published more recently. Consequently, they did not indicate any tendency towards change over time. Traditionally, the Periodontal Index (PI) has been the method of choice for assessing the prevalence and severity of periodontal disease in epidemiological studies. It was assumed that this may have lead to an overestimation of the periodontal destruction, and that the postulated improvement of the condition partly may be ascribed to methods of analysis rather than reality. The observations seemed to indicate that susceptibility to destructive periodontal disease may not be as universal as was previously thought; the majority of the patients could probably retain a considerable proportion of the supporting bone up in high age by routine prophylaxis and improved home care. On the other hand, bone destruction was a serious problem among a minority of "risk" individuals after the age of 40. This subgroup was estimated to make up about 10% of the population. Similar estimates also appeared in published studies on statistical samples from other populations. The individual resistance to periodontal destruction is probably very important for the marginal bone level. The constituent elements of the resistance concept do, however, need further elucidation.