In serial studies of multiple sclerosis lesions, reductions in magnetization transfer ratio (MTR) are thought to be due to demyelination and axonal loss, with later rises due to remyelination. This study followed serial changes in MTR in acute optic neuritis in combination with clinical and electrophysiological measurements to determine if the MTR changes over time mirror the picture in multiple sclerosis lesions, further validating MTR as a marker of tissue integrity. Twenty-nine patients were recruited who had acute optic neuritis for a median of 13 days (range 7-24 days) since the onset of visual symptoms. A clinical examination and measurement of visual evoked potentials (VEP) was performed on each patient. Their optic nerves were imaged with a fat-saturated fast spin echo (FSE) sequence and a magnetization transfer sequence. Twenty-one had multiple subsequent examinations over the course of 1 year. In addition, 27 control subjects had their optic nerves imaged up to three times over 1 year. A blinded observer segmented the optic nerves from the MTR maps. Lesions were defined on the acute FSE images and, from the coordinates, the ratio of mean lesion MTR : healthy nerve MTR (lesion ratio) was calculated for each dataset. The time-averaged mean MTR in control optic nerves was 47.7 per cent units (pu). In diseased optic nerves, baseline mean MTR was 47.3 pu, with a mean lesion ratio of 0.98. The diseased optic nerve MTR and lesion ratio declined over time with a nadir at about 240 days at a mean MTR value of 44.2 pu and mean lesion ratio of 0.91. Subsequently, diseased optic nerve MTR appeared to rise; after 1 year the diseased optic nerve mean MTR was 45.1 pu (mean lesion ratio 0.93), although the difference was not significant compared with the nadir value. For each 0.01 increase in time-averaged lesion ratio logMAR visual acuity recovery improved by 0.03 (95% CI, 0.002, 0.08, P = 0.02). Time-averaged VEP central field latency was shorter by 6.1 ms (95% CI 1.50.7, P = 0.012) per 1 pu rise in time-averaged diseased optic nerve MTR. The early fall in diseased optic nerve MTR is consistent with demyelination and Wallerian degeneration of transected axons. The late nadir compared with studies of multiple sclerosis lesions may have been due to slow clearance of myelin debris. Remyelination may have influenced subsequent MTR changes. The observations support using MTR to monitor symptomatic demyelinating lesions.