Toughened poly(lactic acid) resin compositions comprising poly(lactic acid) and an impact modifier comprising an ethylene copolymer made from monomers (a) ethylene; (b) one or more olefins of the formula CH2═C(R1)CO2R2, where R1 is hydrogen or an alkyl group with 2-8 carbon atoms and R2 is an alkyl group with 1-8 carbon atoms, such as methyl, ethyl, or butyl; and (c) one or more olefins of the formula CH2═C(R3)CO2R4, where R3 is hydrogen or an alkyl group with 1-6 carbon atoms, such as methyl, and R4 is glycidyl. The ethylene copolymer may further be made from carbon monoxide monomers. The compositions may further comprise one or more ethylene/acrylate and/or ethylene/vinyl ester polymers, ionomers, and cationic grafting agents.