A process for producing viscoelastic polyurethane foams having a density of from 100 g/dm 3 to 300 g/dm 3, in which a) polyisocyanate is mixed with b) polymeric compounds having groups which are reactive toward isocyanates, c) optionally chain extenders and/or crosslinkers, d) catalyst, e) blowing agent comprising water and optionally f) additives at an isocyanate index of from 50 to 95 to give a reaction mixture, the reaction mixture is placed in a on a support and reacted to give the flexible polyurethane foam, wherein the polyisocyanate (a) has an isocyanate content of from 20 to 31.5% and is obtainable by mixing 4,4'-MDI and oligomers of propylene oxide having from 2 to 8 propylene oxide units, optionally carbodiimide of 4,4'-MDI and optionally higher homologues of MDI having more than two rings and wherein the polymeric compounds (b) having groups which are reactive toward isocyanates comprise, based on the total weight of the polymeric compounds (b) having groups which are reactive toward isocyanates, b1) from 15 to 40% by weight of at least one polyalkylene oxide having a hydroxyl number of from 20 to 50 mg KOH/g derived from a trifunctional starter molecule and a proportion of ethylene oxide, based on the content of alkylene oxide, of from 4 to 10% by weight and a proportion of primary OH groups of from 50 to 100%, based on the number of OH groups in compound, b2) from 30 to 70% by weight of at least one polyalkylene oxide having a hydroxyl number of from 20 to 50 mg KOH/g derived from a bifunctional starter molecule and a proportion of ethylene oxide, based on the content of alkylene oxide, of from 4 to 10% by weight and a proportion of primary OH groups of from 50 to 100%, based on the number of OH groups in compound, b3) from 10 to 30% by weight of at least one polyalkylene oxide having a hydroxyl number of from 400 to 800 mg KOH/g derived from a bifunctional to tetrafunctional starter molecule and a proportion of ethylene oxide, based on the content of alkylene oxide, of from 80 to 100% by weight and b4) from 0 to 30% by weight of a further polyalkylene oxide. a) polyisocyanate is mixed with b) polymeric compounds having groups which are reactive toward isocyanates, c) optionally chain extenders and/or crosslinkers, d) catalyst, e) blowing agent comprising water and optionally f) additives at an isocyanate index of from 50 to 95 to give a reaction mixture, the reaction mixture is placed in a on a support and reacted to give the flexible polyurethane foam, wherein the polyisocyanate (a) has an isocyanate content of from 20 to 31.5% and is obtainable by mixing 4,4'-MDI and oligomers of propylene oxide having from 2 to 8 propylene oxide units, optionally carbodiimide of 4,4'-MDI and optionally higher homologues of MDI having more than two rings and wherein the polymeric compounds (b) having groups which are reactive toward isocyanates comprise, based on the total weight of the polymeric compounds (b) having groups which are reactive toward isocyanates, b1) from 15 to 40% by weight of at least one polyalkylene oxide having a hydroxyl number of from 20 to 50 mg KOH/g derived from a trifunctional starter molecule and a proportion of ethylene oxide, based on the content of alkylene oxide, of from 4 to 10% by weight and a proportion of primary OH groups of from 50 to 100%, based on the number of OH groups in compound, b2) from 30 to 70% by weight of at least one polyalkylene oxide having a hydroxyl number of from 20 to 50 mg KOH/g derived from a bifunctional starter molecule and a proportion of ethylene oxide, based on the content of alkylene oxide, of from 4 to 10% by weight and a proportion of primary OH groups of from 50 to 100%, based on the number of OH groups in compound, b3) from 10 to 30% by weight of at least one polyalkylene oxide having a hydroxyl number of from 400 to 800 mg KOH/g derived from a bifunctional to tetrafunctional starter molecule and a proportion of ethylene oxide, based on the content of alkylene oxide, of from 80 to 100% by weight and b4) from 0 to 30% by weight of a further polyalkylene oxide. b1) from 15 to 40% by weight of at least one polyalkylene oxide having a hydroxyl number of from 20 to 50 mg KOH/g derived from a trifunctional starter molecule and a proportion of ethylene oxide, based on the content of alkylene oxide, of from 4 to 10% by weight and a proportion of primary OH groups of from 50 to 100%, based on the number of OH groups in compound, b2) from 30 to 70% by weight of at least one polyalkylene oxide having a hydroxyl number of from 20 to 50 mg KOH/g derived from a bifunctional starter molecule and a proportion of ethylene oxide, based on the content of alkylene oxide, of from 4 to 10% by weight and a proportion of primary OH groups of from 50 to 100%, based on the number of OH groups in compound, b3) from 10 to 30% by weight of at least one polyalkylene oxide having a hydroxyl number of from 400 to 800 mg KOH/g derived from a bifunctional to tetrafunctional starter molecule and a proportion of ethylene oxide, based on the content of alkylene oxide, of from 80 to 100% by weight and b4) from 0 to 30% by weight of a further polyalkylene oxide.