The aim of this study was to identify risk factors for severe periodontal disease progression in a Swedish adult population between the years 1973 and 1988-91. In 1973, a random sample of 474 dentate adults living in Jönköping County was examined clinically and radiographically. A questionnaire on demographic and socio-economic status, general health, and dental care habits was also used. During the years 1988-1991, 361 of the individuals examined in 1973 were re-examined. A total of 506 (6%) teeth or in average 1.4 teeth per subject were lost between the 2 examinations. 4 subjects had become completely edentulous. The mean loss of teeth in the different age groups 20-60 years was 0.2, 0.9, 1.4, 2.3, and 2.6, respectively. The periodontal bone level decreased by age both in 1973 and in 1988 91. The mean annual progression rate was 0.06 mm for all 357 individuals and varied between 0.04 and 0.07 mm per subject in the different age groups. The presence of periodontal disease progression was defined as bone loss of >20% at a proximal site between the 2 examinations. The most prevalent tooth types with bone loss of >20% at proximal sites were the maxillary and mandibular 2nd molars and the 1st maxillary molar, representing a % of 18.0, 12.8, and 13.5, respectively. The degree of association between severe periodontal disease progression and explanatory variables was investigated using logistic regression models. The dependent variable was no progression of periodontal disease or severe periodontal disease progression, i.e., subjects with periodontal bone loss >20% at > or =6 sites. Age was found to be correlated with severe periodontal disease progression by an odds ratio of 1.05 (CI: 1.02-1.07). The frequency of females in the group with severe bone loss was 58% and higher than in the non-progressing group, 50%. Only 9% in the group with no bone loss smoked as compared to 38% in the group of individuals with severe periodontal bone loss. % supragingival plaque, gingival inflammation, and deepened periodontal pockets (> or =4 mm) at baseline were related to severe periodontal disease progression by odds ratios of 1.03 (CI: 1.02-1.05), 1.01 (1.00-1.03), and 1.03 (1.00-1.05), respectively. In the multivariate logistic regression model, age (odds ratio 1.13 (CI: 1.06-1.19)), smoking (odds ratio 20.25 (5.07-80.83)), and % pockets > or =4 mm (odds ratio 1.15 (1.04-1.27)) remained significantly associated with severe disease progression. Furthermore, female gender and differences in income level appeared in the multivariate analysis to be related with severe bone loss, with odds ratios of 3.19 (CI: 1.02-9.97) and 8.46 (CI: 1.97-36.37), respectively.