NAION in both eyes has been reported in as few as 10.5 percent and as many as 73 percent of patients. In a study of bilateral NAION where all patients were prospectively logged, Beri et al. reported that 17.5 percent of patients developed bilateral disease at 1 year of followup and 34.5 percent developed it at 5 years. However, Beck et al., using a life table analysis on the same cohort reported by Beri et al., estimated the risk of bilateral NAION to be 12 percent within 2 years and 19 percent within 5 years. The IONDT has so far similarly reported a 12 percent (25/216) incidence of bilateral NAION in its randomized patients. The incidence in the nonrandomized group, 91 percent of whom had visual acuity better than 20/64, is much lower at 4 percent (5/136).