Improved low density polyurethane foams are prepared from an isocyanate-terminated prepolymer having an NCO content of more than about 31 to about 33.5% by weight comprising:(a) a polyphenylene polymethylene polyisocyanate comprising(i) about 30 to about 100% by weight of diphenylmethane diisocyanate and(ii) the remainder selected from the group consisting essentially of higher homologues of polyphenylene polymethylene polyisocyanate, isocyanate-containing ester groups, urea groups, biuret groups, aliphatic groups, carbodiimide groups, isocyanurate groups, uretdione groups and urethane groups; and(b) a polyoxyalkylene polyol having an oxyethylene content of about 30 to about 90%, a molecular weight of about 1000 to about 12000 and a functionality of about 2 to about 8.