AIM: To analyze the effect of systemic high-dose corticosteroid on the choroid in patients with unilateral optic neuritis. METHODS: A retrospective comparative cohort study. Seventy-six eyes of 38 patients with unilateral optic neuritis that received systemic high-dose corticosteroid treatment were enrolled. Choroidal thickness (CT) and choroidal vascularity index (CVI) were measured in both affected and the fellow eyes at baseline, 1wk, 1 and 3mo. Changes in CT and CVI were analyzed in both eyes and compared between eyes. RESULTS: The mean CT and CVI were 349 μm and 0.70 in the affected eyes and 340 μm and 0.69 in the fellow eyes at baseline (P=0.503 and 0.440, respectively). Decrement of CT and CVI at month 3 were significant in affected eyes (P=0.017 and P<0.001). Decreased CVI began 2wk after treatment whereas CT decreased from 1mo. The CVI also decreased significantly in fellow eyes at 3mo compared to the baseline (P=0.001). CONCLUSION: A significant decrement in CT and CVI can appear after 3mo in optic neuritis patients treated with high-dose systemic corticosteroid treatment. The decrease in CVI appeared earlier than the decrease in CT, suggesting choroidal vasoconstriction caused by systemic steroid as a possible mechanism.