Aqueous polymer dispersion containing a water-dispersed polyurethane polymer and a vinyl (preferably acrylic) polymer in a weight ratio of from 30/70 to 5/95, wherein said polyurethane polymer is the reaction product of: A) an isocyanate-terminated polyurethane prepolymer having an acid value of ≧44 mg KOH/g of prepolymer (solids) and formed from reactants comprising an organic polyisocyanate component and an organic poly(isocyanate-reactive) group) component in which the isocyanate-reactive groups are selected from —OH and optionally one or more of —NH2, —NH—, and —SH (usually a polyol), wherein i) at least 70 weight % of said polyisocyanate component is constituted by at least one aliphatic polyisocyanate, ii) said poly(isocyanate-reactive group) component includes at least one acid-bearing poly(isocyanate-reactive group) compound for providing anionic groups which provide or contribute to water-dispersibility, iii) the reactants are used in amounts corresponding to a ratio of isocyanate groups to isocyanate-reactive groups (usually hydroxyl) within the range of 1.4/1 to 2.9/1; and B) an active hydrogen chain-extending compound(s).