The effect of a 30 day course of corticotrophin gelatin injections on the natural history of acute unilateral optic neuritis in 27 patients is reported. A further 27 patients received inert injections in a double-blind control. Measurements were made on visual acuity, visual field, macular threshold, and colour vision, and the development of optic atrophy and other neurological episodes was recorded. No significant differences were found between treated and control groups at intervals up to two years from the start of the trial. The criteria for inclusion of patients are given and it is concluded that corticotrophin has no effect on the natural history of the condition as defined.