The goal of this study was to determine the prevalence and incidence of periodontal disease and its relationship with non-insulin-dependent diabetesmellitus (NIDDM). Two thousand two hundred seventy-three Pima Indians (949 men, 1324 women) aged ≥15 yr from the Gila River Indian Community in Arizonawere examined between 1983 and 1989. Periodontal disease was diagnosed by tooth loss and by percentage of interproximal crestal alveolar bone loss ascertained from panoramic radiography. Subjects with little or no evidence of periodontal disease were classified as nondiseased. Thus, the incidence of advanced periodontal disease was determined. The age- and sex-adjusted prevalence of periodontal disease at first dental examination was 60% in subjects with NIDDM and 36% in those without. Twenty-two new cases developed in a subset of 701 subjects (272 men, 429 women) aged 15–54 yr who initially had little or no evidence of periodontal disease and had at least one additional dental examination. The incidence of periodontal disease in this group was similar in men and women (incidence-rate ratio 1.0, 95% confidence interval [Cl] 0.5-1.9, controlled for age and diabetes). Higher age predicted a greater incidence of periodontal disease (χ2 = 30.6, df = 3, P < 0.001, controlled for sex and diabetes). The rate of periodontal disease in subjects with diabetes was 2.6 times (95% Cl 1.0–6.6, controlled for age and sex) that observed in those without. Although periodontal disease was common in nondiabetic Pima Indians, in whom most of the incident cases occurred, diabetes clearly conferred a substantially increased risk. Thus, periodontal disease should be considered a nonspecific complication of NIDDM.