The invention relates to a process for the preparation of CFC-free, flexible polyurethane foams or molded foams by reactingA) liquid polyisocyanate mixtures containing bonded urethane groups and having a content of NCO groups of from 20 to 30% by weight, these mixtures themselves being prepared by reacting a mixture of diphenylmethane diisocyanates and polyphenyl-polymethylene polyisocyanates (crude diphenylmethane diisocyanate) with a polyether-polyol mixture, having a hydroxyl number of from 120 to 25, of at least one polyoxypropylene glycol and at least one trifunctional polyoxypropylene-polyoxyethylene glycol having a content of ethylene oxide units of from 90 to 10% by weight or by reacting 4,4'-diphenylmethane diisocyanate (diphenylmethyl diisocyanate) or an diphenylmethane diisocyanate isomer mixture with said polyether-polyol mixture to give an NCO-containing quasi-prepolymer and diluting this quasi-prepolymer with crude diphenylmethane diisocyanate, withB) relatively high-molecular-weight polyhydroxyl compounds.