The present invention is directed to a process for preparing an aqueous polyisocyanate dispersion which has an isocyanate content of at least 0.5% by weight bya) introducing an aqueous medium into a static mixer,b) introducing a polyisocyanate into the static mixer,c) mixing the polyisocyanate with the aqueous medium in the static mixer andd) withdrawing the aqueous polyisocyanate dispersion from the outlet of the static mixer,e) introducing the aqueous polyisocyanate dispersion into the inlet of a second static mixer,f) introducing into the same of a different inlet of the second static mixer a chain extender containing at least a primary or secondary hydroxy monoamine and/or a polyamine having primary and/or a polyamine having primary and/or secondary amino groupsg) mixing the chain extender with the aqueous polyisocyanate dispersion in the second static mixer,h) withdrawing the reacting mixture of the aqueous polyisocyanate dispersion and the chain extender from the outlet of the second static mixer andi) allowing the polyisocyanate and chain extender to react to form the aqueous modified polyisocyanate dispersion.