A prospective study was conducted in 438 patients with anterior ischemic optic neuropathy (AION). There were 388 patients with nonarteritic AION and 50 with arteritic AION. The risk of bilaterality in patients with arteritic AION was found to be 1.9 times the risk in patients with nonarteritic AION (P = 0.0118). The cumulative incidence curve, considering the time taken to develop bilateral AION for nonarteritic cases was significantly (P = 0.0103) different from that for arteritic cases. The estimated 25th-percentile time to development of bilateral AION was much shorter in patients with arteritic AION (0.4 month) than in those with nonarteritic AION (32.4 months). In arteritic AION, unilateral as well as bilateral AION had almost invariably developed before systemic steroid therapy was started and not after, indicating that this therapy is effective in preventing the development of AION in giant cell arteritis. In nonarteritic AION, the risk of bilaterality was significantly greater in men (P = 0.0113) and in young (less than 45 years old) patients with diabetes (P = 0.0245), with no significant difference attributable to the other age groups or other associated systemic diseases. In this study, it was found that young diabetic men have a risk of AION developing in the second eye that is 1.56 times the risk in young diabetic women, 2.56 times the risk in women who either are nondiabetic or are not young, and 1.64 times the risk in both older men and nondiabetic men.