An aqueous crosslinking resin composition comprising (I) an aqueous resin dispersion obtained by emulsion polymerization of a monomer mixture consisting of (a) 2 to 50% by weight of an unsaturated carboxylic acid monomer, (b) 0.5 to 60% by weight of a carbonyl-containing unsaturated monomer containing at least one carbonyl group based on an aldo group or a keto group, and (c) the remainder of at least one monomer(s) selected from the group consisting of alkyl acrylates or methacrylates having from 1 to 8 carbon atoms in the alkyl moiety thereof, glycidyl methacrylate, glycol mono- or diacrylates, glycol mono- or dimethacrylates, aromatic vinyl compounds, vinyl halides, vinylidene chloride, acrylonitrile, methacrylonitrile, saturated carboxylic acid vinyl esters, unsaturated carboxylic acid amides, N-alkyl and/or N-alkylol compounds of unsaturated carboxylic acid amides, unsaturated sulfonic acids, and hydroxyl-containing unsaturated compounds, and (II) a hydrazine compound having at least two hydrazino groups (--NHNH.sub.2) per molecule, with the molar ratio of the hydrazino group in said hydrazine compound (II) to the carbonyl group in the resin of said aqueous resin dispersion (I) being from 0.05 to 5, and with the resin content in the aqueous resin dispersion (I) having been solubilized to a degree of at least 5% by weight by addition of sufficient amount of an alkali and/or water miscible organic solvent. The resin composition easily undergoes a crosslinking reaction at room temperature to provide a crosslinked film having excellent adhesion to a substrate, well-balanced resistance to water and solvents, excellent whitening resistance, and hardness.