A radiation-curable coating composition adapted for the coating of optical glass fiber, comprising an ethylenically unsaturated polyurethane which is the reaction product of:(A) a polyether diol having a molecular weight in the range of 200 to 8,000 and a structural unit represented by the formula: ##STR1## in which R.sup.1 -R.sup.4 are each independently selected from hydrogen and an alkyl group containing from 1-4 carbon atoms, such as a polymer of copolymer of methyl tetrahydrofuran;(B) a polyisocyanate compound, preferably a diisocyanate; and(C) a polymerizable ethylenically unsaturated compound having a single hydroxy group, such as a monohydric acrylate;there being employed per equivalent of hydroxy in component (C) from 1.1 to 2 equivalents of said polyisocyanate with the balance of the isocyanate groups being reacted waith the hydroxy groups of said diol.