The acute effects of an oral hypocalcemic substance, Na2HPO4/KH2PO4, was tested on visual function in six multiple sclerosis patients with optic nerve dysfunction. Marked improvement in visual function testing occurred in five patients within one hour after phosphate ingestion and generally reversed over one to two hours. Improvement was correlated with elevation of serum phosphate and chemical and clinical evidence of serum calcium lowering. Placebo had no effect on visual function in one multiple sclerosis patient and phosphate had no effect on pupillometry in a normal control subject. Though the results do not represent a practical treatment, they do show the dramatic potential for acute pharmacologic modification of signs and symptoms of multiple sclerosis.