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This study reports the prospective follow–up of a cohort of patients with acute optic neuritis examined with serial visual tests, visual evoked potentials (VEPs), conventional and triple–dose gadolinium (Gd)–enhanced magnetic resonance imaging (MRI) to examine which factors are important in visual recovery. Thirty–three patients were recruited with acute unilateral optic neuritis. A clinical and VEP assessment was performed on each. Optic nerve MRI was performed using fast spin echo (FSE) (on all) and triple-dose Gdenhanced T1–weighted sequences (n = 28). Optic nerve lesion lengths were measured. Serial assessments were performed on 22 of the patients up to one–year. Serial Gd–enhanced optic nerve imaging was performed on 15 of the patients until enhancement ceased. The final 30–2 Humphrey visual field mean deviation (MD) was 2.55 dB higher in patients in the lowest quartile of initial Gd–enhanced lesion length compared with the other quartiles (p < 0.01) but recovery was not related to the duration of enhancement. The initial recovery of Humphrey MD was 4.60 dB units per day in patients with good eventual recoveries (MD > –6.0 dB) and 0.99 dB per day in poor-recovery patients (p = 0.02).Good–recovery patients had mean central field VEP amplitudes 2.29 µV higher during recovery than poor-recovery patients (p = 0.047). The results suggest that factors which are associated with a better prognosis are: having a short acute lesion on triple–dose gadolinium enhanced imaging, higher VEP amplitudes during recovery and a steep gradient of the initial improvement in vision. [ABSTRACT FROM AUTHOR]