The invention relates to a process for the batch or semi-batch preparation of a polymer polyol, which process involves (1) preparing a seed by polymerizing in a base polyol at least one ethylenically unsaturated monomer in the presence of a polymerization initiator and macromer, and in the presence of from 0.5 to 50% wt of polymer polyol heel, based on amount of polymer present in the heel compared with total amount of polymer formed, (2) adding additional ethylenically unsaturated monomer to the seed obtained in step (1) and optionally additional polyol, and (3) polymerizing the mixture obtained in step (2).