OBJECTIVE - Infection may be a type 2 diabetes risk factor. Periodontal disease is a chronic infection. We hypothesized that periodontal disease was related to A1C progression in diabetes-free participants. RESEARCH DESIGN AND METHODS - The Study of Health in Pomerania (SHIP) is a population-based cohort in Germany including 2,973 diabetes-free participants (53% women; aged 20-81 years). Participants were categorized into four groups according to increasing baseline periodontal disease levels (percentage of sites per mouth with attachment loss ≥5 mm, determined a priori); sample sizes for each respective category were 1,122, 488, 463, and 479 (241 participants were edentulous). Mean absolute changes (year 5 minus baseline) in A1C (ΔA1C) were regressed across periodontal categories while adjusting for confounders (e.g., age, sex, smoking, obesity, physical activity, and family history). RESULTS - Across baseline periodontal disease categories, ΔA1C ± SEM values were 0.023 ± 0.02, 0.023 ± 0.02, 0.065 ± 0.03, and 0.106 ± 0.03 (Ptrend = 0.02), yielding an approximate fivefold increase in the absolute difference in ΔA1C when dentate participants in the highest versus lowest periodontal disease category were compared; these results were markedly stronger among participants with high-sensitivity C-reactive protein ≥1.0 mg/l (Pinteraction = 0.01). When individuals who had neither baseline periodontal disease nor deterioration in periodontal status at 5 years were compared with individuals with both poor baseline periodontal health and longitudinal periodontal deterioration, mean ΔA1C values were 0.005 vs. 0.143% (P = 0.003). CONCLUSIONS - Periodontal disease was associated with 5-year A1C progression, which was similar to that observed for a 2-SD increase in either waist-to-hip ratio or age in this population.