The present invention relates to a method for producing flexible polyurethane foams, wherein an isocyanate component (component B) which comprises fatty acid derivatives comprising hydroxyl groups is used as starting substance. The flexible polyurethane foams according to the invention have a bulk density according to DIN EN ISO 3386-1-98 in the range of ≧10 kg/m3 to ≦150 kg/m3, preferably ≧20 kg/m3 to ≦70 kg/m3, and in general their compressive strength according to DIN EN ISO 3386-1-98 is in the range of ≧0.5 kPa to ≦20 kPa (at 40% deformation and 4th cycle). The invention also provides an NCO-terminated prepolymer comprising urethane groups obtainable by reaction of one or more polyisocyanates (B1) with one or more fatty acid derivatives comprising hydroxyl groups (B2).