The present invention relates to aqueous polyurethane dispersions wherein the polyurethanes contain terminal hydrazide groups and correspond to the formula wherein the terminal, modified hydrazide groups are present in an amount of 1 to 25% by weight [calculated as C(O)—NH—NH—C(O), MW 84], based on the weight of the polyurethanes, and wherein X represents OR′ or NHR′, R represents the residue obtained by removing the isocyanate groups from an NCO prepolymer, R′ represents a group which is inert to Isocyanate groups under the conditions used to form the polyurethane of in formula I, R″ represents a divalent, linear or branched aliphatic group containing 2 to 10 carbon atoms, provided that there are at least two carbons between the oxygen atoms, wherein the aliphatic group may optionally be substituted by heteroatoms to form ether or ester groups, m is 0 or 1 and n is 2 to 4.