OBJECTIVE - Adverse microbial exposures might contribute to diabetogenesis. We hypothesized that clinical periodontal disease (a manifestation of microbial exposures in dysbiotic biofilms) would be related to insulin resistance among diabetes-free participants. The roles of inflammatory mediation and effect modification were also studied. RESEARCH DESIGN AND METHODS - The continuous National Health and Nutrition Examination Survey 1999-2004 enrolled 3,616 participants (51% women) who received a periodontal examination and fasting blood draw. Participants were mean age (±SD) 43±17 years and 28% Hispanic, 52% Caucasian, 17% African American, and 3%other. Log-transformed values of the homeostasismodel assessment of insulin resistance (HOMA-IR) or HOMA-IR ≥3.30 (75th percentile) were regressed across full-mouth periodontal probing depth (PD) levels using linear and logistic models. White blood cell (WBC) count and C-reactive protein (CRP) were considered as either mediators or effect modifiers in separate analyses. Risk ratios (RRs) stem from marginal predictions derived from the logistic model. Results were adjusted for multiple periodontal disease and insulin resistance risk factors. RESULTS - In linear regression, geometric mean HOMA-IR levels increased by 1.04 for every 1-mm PD increase (P = 0.007). WBC mediated 6% of the association (P < 0.05). Among participants with WBC≤6.4×109, PD was unrelated to HOMA-IR ≥3.30. Fourth-quartile PD was associated with HOMA-IR ≥3.30 among participants with WBC >7.9 × 109; RR 2.60 (1.36-4.97) (P for interaction = 0.05). Findings were similar among participants with CRP .3.0 mg/L (P for interaction = 0.04). CONCLUSIONS - Periodontal infection was associated with insulin resistance in a nationally representative U.S. sample of diabetes-free adults. These data support the role of inflammation as both mediator and effect modifier of the association.