Objective: Previous studies have identified a higher prevalence of diabetes mellitus (DM) among patient cohorts with nonarteritic anterior ischemic optic neuropathy (NAION). We sought to determine the development of incident NAION among a group of newly diagnosed patients with DM and to estimate the incidence of NAION among the elderly. Design: Medicare 5% database study. Participants: A total of 25 515 patients with DM and an equal number of age- and gender-matched nondiabetic patients. Methods: Query of Medicare 5% claims files identified patients with a new diagnosis of DM in 1994. A randomly selected control group was created using 1-to-1 propensity score matching. Patients with a diagnosis of giant cell arteritis, preexisting DM, and age 68 years or older or >95 years were excluded. Patients with DM and controls were followed for the development of NAION over the following 4745 days. Main Outcome Measures: Incidence of NAION among patients with and without DM. Results: In each group, 85% were white, 11% were black, and 4% were other race. Patients were aged 76.4 years, and 40% were male. Mean follow-up was 7.6 years. In the diabetes group, 188 individuals developed NAION (0.7%) compared with 131 individuals (0.5%; P < 0.01) in the control group. In unadjusted Cox regression analysis, having DM was associated with a 43% increased risk (hazard ratio [HR]: 1.431; 95% confidence interval [CI], 1.1451.789) of developing NAION. After adjusting for other covariates, the risk of developing NAION among individuals with DM was reduced to 40% (HR 1.397; 95% CI, 1.1151.750). Male gender increased an individual's risk of developing NAION by 32% (HR 1.319; 95% CI, 1.0521.654). No other covariate was statistically significantly associated with developing NAION. The annual incidence of NAION was 82 per 100 000 persons. Conclusions: Diabetes mellitus significantly increased the risk of the diagnosis NAION. The incidence of NAION among patients aged more than 67 years may be higher than previously reported. Financial Disclosure(s): The author(s) have no proprietary or commercial interest in any materials discussed in this article.