OBJECTIVE Shortened leukocyte telomere length (LTL) and diagnosis of periodontitis are associated with an increased risk of complications and mortality in diabetes. This study investigated the association between LTL, endotoxemia, and severity of periodontitis in a large cohort of people with diabetes. RESEARCH DESIGN AND METHODS Six hundred thirty individuals (371 with type 2 and 259 with type 1 diabetes) were recruited from the University College Hospital in London, U.K. During a baseline visit, blood was collected for standard biochemical tests and DNA extraction, while a dental examination was performed to determine diagnosis and extent of periodontitis. LTL was measured by real-time PCR, and endotoxemia was assessed by the limulus amoebocyte lysate method. RESULTS Two hundred fifty-five individuals were diagnosed with gingivitis, 327 with periodontitis (114 with moderate and 213 with severe disease), and 48 with edentulous. Diagnosis of periodontitis was associated with shorter LTL (P = 0.04). A negative association between LTL and endotoxemia was found in the severe periodontitis and type 2 diabetes groups (P = 0.01 for both). Shorter LTL was associated with increased extent of periodontitis (P = 0.01) and increased insulin resistance (homeostatic model assessment). Multiple adjustments for biochemical, anthropometric, and medication-use variables did not affect the results. CONCLUSIONS LTL is associated with endotoxemia and diagnosis of periodontitis in people with diabetes. LTL shortening might represent a novel biological pathway accounting for previous epidemiological data that documented higher prevalence of diabetes and its complications in people with periodontitis and vice versa.