OBJECTIVE To explore the clinical value of magnetic resonance imaging (MRI) in optic neuritis. METHODS Forty patients with optic neuritis were examined with the spectral presaturation inversion recovery and fluid attenuated inversion recovery. RESULTS The lesion was frequently seen in orbital portion (86.7%) and intracanalicular portion (29.3%), and primarily in the axial part (49.3%) and transverse section (38.7%) of the optic nerve. The longitudinal extent of lesion was from 2 to 35 mm (mean, 12.6 mm), the visual acuity of patients with optic neuritis was related to the site of the lesion, and negatively correlated with the longitudinal extent of lesion of logarithm curve. A poor visual acuity (< 0.01) was significantly associated with lesion of greater extent > 10 mm and the transverse or axial lesion, but the good visual acuity (>or= 0.3), with the lesion of smaller extent < 10 mm and the axial or peripheral lesion. In addition, paramagnetic contrast-medium, Gd-DTPA, enhancement scanning was carried out in 11 cases of optic neuritis, of them the visual acuity was increased in 10 cases after administration of corticosteroid, and in one case, visual improvement was not seen. CONCLUSIONS MRI is a new and useful equipment which is helpful in the diagnosis and treatment of optic neuritis.