Disclosed is a method of strengthening keratinous fibers, the method comprising contacting the fibers with a strengthening-effective amount of a composition comprising a poly(vinylamine-vinylformamide) copolymer and a carrier and, optionally, styling the fibers treated with the composition, wherein the poly(vinylamine-vinylformamide) copolymer comprises a high molecular weight poly(vinylamine-vinylformamide) copolymer, a low molecular weight poly(vinylamine-vinylformamide) copolymer, or combination thereof, and wherein the high or low poly(vinylamine-vinylformamide) copolymer comprises at most about 95 mol percent vinylamine monomers. Also disclosed is a method of strengthening keratinous fibers which have been damaged by chemical contact, mechanical stress or heat by contacting the fibers with a composition comprising a poly(vinylamine-vinylformamide) copolymer and a carrier.
