OBJECTIVE - Type 2 diabetes and periodontal disease are known to be associated, but the temporality of this relationship has not been firmly established. We investigated whether baseline periodontal disease independently predicts incident diabetes over two decades of follow-up. RESEARCH DESIGN AND METHODS - A total of 9,296 nondiabetic male and female National Health and Nutrition Examination Survey (NHANES I) participants aged 25-74 years who completed a baseline dental examination (1971-1976) and had at least one follow-up evaluation (1982-1992) were studied. We defined six categories of baseline periodontal disease using the periodontal index. Of 7,168 dentate participants, 47% had periodontal index = 0 (periodontally healthy); the remaining were classified into periodontal index quintiles. Incident diabetes was defined by 1) death certificate (ICD-9 code 250), 2) self-report of diabetes requiring pharmacological treatment, or 3) health care facility stay with diabetes discharge code. Multiva- riable logistic regression models assessed incident diabetes odds across increasing levels of periodontal index in comparison with periodontally healthy participants. RESULTS - The adjusted odds ratios (ORs) for incident diabetes in periodontal index categories 1 and 2 were not elevated, whereas the ORs in periodontal index categories 3 through 5 were 2.26 (95% CI 1.56-3.27), 1.71 (1.0-2.69), and 1.50 (0.99-2.27), respectively. The ORin edentulous participants was 1.30 (1.00-1.70). Dentate participants with advanced tooth loss had an OR of 1.70 (P < 0.05) relative to those with minimal tooth loss. CONCLUSIONS - Baseline periodontal disease is an independent predictor of incident diabetes in the nationally representative sample of NHANES I.