Purpose: The aim of this study was to assess risk indicators for reduced marginal bone level in the individual, with emphasis on apical periodontitis. Materials and Methods: Six hundred and sixteen randomly selected Danish adults (304 women and 312 men), mean age of 42 years, underwent a full-mouth radiographic survey. The marginal bone level was measured from the cemento-enamel junction to the marginal bone. The measurements were performed at the mesial (Am) and distal (Ad) aspect of the tooth. The marginal bone level for each individual was calculated: Aind = Σ Ateeth/nteeth, and Aind >= 4 mm was considered as reduced marginal bone level. The periapical status was assessed by the periapical index (PAI), which was dichotomised (healthy = PAI scores 1 and 2, and diseased = PAI scores 3, 4 and 5). Coronal restorations (crowns, fillings and inlays) and smoking status were also recorded. All variables were analysed in a logistic regression model with reduced marginal bone level as the outcome. Results: The impact of age (odds ratio (OR) = 3.3), smoking (OR = 10.5) and apical periodontitis (OR = 4.7) on reduced marginal bone level was statistically significant (p 0.01). Conclusions: Not surprisingly, this study showed that smoking and older age were risk indicators for having reduced marginal bone level >= 4 mm. Even when adjusted for these factors, individuals with >= 1 tooth with apical periodontitis were five times more at risk of having a reduced marginal bone level than those with no periapical infection.