Progression of periodontal disease in adult subjects in the absence of periodontal therapy was monitored in 2 populations. One group of 64 Swedish subjects (mean age 40.5 years at entry) with mild to moderate periodontal attachment loss was monitored for attachment level changes at baseline, 3 and 6 years. A second group of 36 Americans (mean age 34.3 years at entry) with advanced destructive periodontal disease was monitored for attachment level changes at baseline and 1 year. Of 4101 sites examined at baseline and at 3 years in the Swedish subjects, only 158 sites (3.9%) showed attachment loss of more than 2 mm. No measurable change was found in 1440 sites (35.1%). Of 4097 sites examined at 3 and 6 years, 67 sites (1.6%) showed attachment loss greater than 2 mm; 57.4% of sites showed no measurable change; and 19 sites (0.5%) showed a decrease in probeable attachment level of more than 2 mm. During the 6-year interval, 523 sites (11.6%) showed attachment loss of more than 2 mm; 20% of sites showed no measurable change and 11 sites (0.2%) showed more than 2 mm of attachment "gain". Approximately 50% of sites that showed no measurable change in the first 3-year period showed loss in the next 3 years. In contrast, of the sites which showed some level of attachment loss in the first period, approximately 2/3 showed no loss in the second monitoring period. Of 3210 sites monitored in the American subjects, 102 sites (3.2%) exhibited more than 2 mm of additional attachment loss; 26% of sites showed no measurable change and 138 sites (4.3%) showed a decrease in probeable attachment level of more than 2 mm. The association between attachment level changes and initial attachment level (baseline) was examined by regression analysis. The slopes of the regression lines for both populations were not consistent with a hypothesis that sites with more advanced attachment loss are more prone to additional destruction, in the absence of treatment, than sites with initially less attachment loss. In the American group, some sites with initially advanced attachment loss, exhibited a decrease in probing attachment level.