Invention relates to a process for producing viscoelastic flexible polyurethane foams by reacting a) polyisocyanates with b) compounds having at least two hydrogen atoms which are reactive toward isocyanate groups, wherein as b) a mixture of bi) from 25 to 70% by weight of a hydroxyl-comprising natural oil or fat or a reaction product of a hydroxyl-comprising natural oil or fat with alkylene oxides, bii) from 3 to 30% by weight of at least one polyether having a hydroxyl number of from 100 to 800 mg KOH/g and a functionality of 3-5 selected from the group consisting of polyether prepared by addition of alkylene oxides onto an amine, biii) 20-50% by weight of at least one polyether alcohol having a hydroxyl number of from 10 to 80 mg KOH/g and a functionality of 2-5, where the proportion of ethylene oxide is 5-25% by weight, based on the weight of the polyether alcohol, and at least part of the ethylene oxide is added on at the end of the polyether chain, biv) >0-8% by weight of at least one polyether alcohol having a functionality of 2-4, a hydroxyl number of 15-60 mg KOH/g and a content of ethylene oxide of >50% by weight of the polyether alcohol is used.