Abstract: Flexible foams based on polyisocyanate polyaddition products are produced by reacting polyisocyanates with compounds which are reactive toward isocyanates in the presence of catalysts, blowing agents and, if desired, flame retardants, auxiliaries and/or additives, wherein polyether polyalcohols (c) which are obtainable by addition of alkylene oxides onto substances containing active hydrogens are used as substances which are reactive toward isocyanates, where, in a first stage, at least one substance (a) containing 2 or 3 active hydrogens is reacted with at least one alkylene oxide in the presence or absence of catalysts, subsequently at least one substance (b) which contains at least four active hydrogens and, if desired, further catalyst is added to the reaction mixture from the first stage and this mixture is then reacted in a second stage with at least one alkylene oxide.