['To evaluate the usefulness of the signal intensity ratio (SIR) of the optic nerve to the white matter (WM) on short tau inversion recovery (STIR) images to diagnose acute optic neuritis (AON).', 'The 405 consecutive patients with suspected orbital diseases underwent orbital magnetic resonance imaging (MRI) using a 3-T scanner between June 2008 and August 2011. Among them, 108 optic nerves (33 AON and 75 control) were retrospectively analysed. The averaged SIR (SIRave) and maximum SIR (SIRmax) were defined as the averaged signal intensity (SI) of the optic nerve divided by that of WM, and the maximum SI of the optic nerve divided by averaged SI of WM, respectively. These values were compared between AON and control using the Mann-Whitney U test. A P\u2009<\u20090.05 was considered statistically significant.', 'SIRave and SIRmax were significantly (P\u2009<\u20090.001) higher in the AON compared to the control. At a cut-off SIRave value of 1.119, the sensitivity, specificity and accuracy were 0.939, 0.840, and 0.870; and at a cut-off SIRmax value of 1.281, these were 1.000, 0.720 and 0.806, respectively.'] Is the signal intensity ratio of the optic nerve to ipsilateral frontal white matter of value in the diagnosis of acute optic neuritis?