PURPOSE: To obtain a soft polyurethane foam low in Shore A hardness and compression set by reacting a specific prepolymer with a specific polyol.

CONSTITUTION: A prepolymer comprising a reaction product of 25-75 wt.% stoichiometrically excess diphenylmethane diisocyanate (derivative) and 25-75 wt.% first polyol of a polyether polyol having not less than 1,000 equivalent weights and 1.6-3 nominal functional values is reacted with a second polyol having not less than 1,500 equivalent weights.