Fifty subjects will be enrolled in this Phase II, investigator-initiated, randomized and blinded cross-over trial of dalfampridine of 8 weeks duration (Table 1). The study will test the hypothesis that dalfampridine, when administered to subjects with incomplete visual recovery after optic neuritis from MS, will result in symptomatic improvement in visual function. The study will consist of one screening/baseline visit, one visit during treatment with active drug, and one visit on placebo. After the baseline visit, subjects will be randomly assigned to receive study medication or placebo for the first three weeks, followed by a two week wash-out, and then treatment reallocation for the latter three weeks.