This invention relates to polymer polyols comprising the free-radical polymerization product of a base polyol, at least one ethylenically unsaturated monomer, and, optionally, a preformed stabilizer, in the presence of at least one free-radical polymerization initiator and at least one chain transfer agent, in which the base polyol is a natural oil. A process for preparing these polymer polyols is also described. The present invention also relates to a polyurethane foam prepared from these polymer polyols and to a process for the preparation of these polyurethane foams.