BACKGROUND: Interpretation of risk for periodontitis is critical for treatment planning. How periodontists assess risk for periodontitis is unclear. PURPOSE: To study (1) what factors periodontists use when assessing the risks for worsening periodontal conditions anticipating that no treatment would be provided, and (2) if risk assessment is consistent and independent of specialty background training. MATERIAL AND METHODS: Medical history, clinical dental data, full-mouth intra-oral radiographs, and slide pictures were obtained from each of 51 subjects, and the information was provided to 23 examiners. RESULTS: The mean age of the study subjects was 51.5 years (SD +/- 17.7, range 23-81), with 28 women included. In 10 of the subjects, only gingivitis was identified, while 22 subjects had advanced chronic periodontitis. Risk scores assigned for 2 and 4 years differed significantly between European- and US-trained periodontists (p < 0.001) and between graduate students in training and periodontists from either the US or Europe (p < 0.01) (Wilcoxon n-pair test), with European periodontists scoring the lowest risks. Risk scores were correlated between groups (p < 0.01 with rho range 0.82-0.89) (Spearman’s rank correlation). The best-fit model (r2 = 0.86) to assess perceived risk for worsening periodontal conditions based on data from all examiners combined included the following variables: (1) overall horizontal alveolar bone loss (p < 0.000), (2) age-adjusted proportional radiographic bone height score for the worst site (p < 0.000), and (3) proportion of pocket probing depths > or = 6.0 mm. CONCLUSIONS: Differences exist on the scale of risk values based on specialty training. Consistency in scoring patterns exists. The examiners based their assigned risk scores almost exclusively on measures of existing disease severity, including radiographic bone loss and numbers of periodontal pockets > or = 6.0 mm, and excluding most known risk factors such as smoking, diabetes, and poor oral hygiene.