A process for making a polymer polyol that contains styrene/acrylonitrile copolymer particles dispersed in a continuous phase that includes at least one polyol, comprising the steps of a) forming a seed dispersion containing 5 to 25 weight-% of an unsaturated macromer, 2.5-20 weight-% of styrene polymer or copolymer particles having a particle size of 50 to 500 nm, and 45-92.5 weight-% of a seed dispersion carrier polyol, the weight-percentages being based on the weight of the seed dispersion; b) forming a carrier mixture containing the seed dispersion, a base polyol and a solvent, and dispersing a monomer mixture that includes at least 25 weight-% styrene and greater than 20 weight-% acrylonitrile, based on the weight of the monomer mixture, into the carrier mixture, wherein i) the amount of seed dispersion is sufficient to provide 0.025 to 0.35 weight percent seed particles and 0.1 to 0.75 weight percent of unsaturated macromer based on the combined weight of the carrier mixture and monomer mixture; ii) the weight of the monomer mixture is at least 200 times the weight of the seed particles and when polymerized is sufficient to provide the polymer polyol with at least 30 weight-percent dispersed polymer particles and iii) the amount of the solvent is 6 to 20% of the combined weight of the carrier mixture and the monomer mixture c) polymerizing the dispersed monomer mixture to form polymer particles dispersed in the carrier mixture and then d) removing the solvent from the carrier mixture to form a polymer polyol product containing at least 30 weight-percent dispersed polymer particles, wherein the dispersed polymer particles have a particle size of 1 to 3 μm and a particle size span of less than 1.25 wherein the particle size is measured according to ASTM D1921 using laser diffraction and is the mode particle size, and wherein the particle size span is (D 90 -D 10)/D 50. a) forming a seed dispersion containing 5 to 25 weight-% of an unsaturated macromer, 2.5-20 weight-% of styrene polymer or copolymer particles having a particle size of 50 to 500 nm, and 45-92.5 weight-% of a seed dispersion carrier polyol, the weight-percentages being based on the weight of the seed dispersion; b) forming a carrier mixture containing the seed dispersion, a base polyol and a solvent, and dispersing a monomer mixture that includes at least 25 weight-% styrene and greater than 20 weight-% acrylonitrile, based on the weight of the monomer mixture, into the carrier mixture, wherein i) the amount of seed dispersion is sufficient to provide 0.025 to 0.35 weight percent seed particles and 0.1 to 0.75 weight percent of unsaturated macromer based on the combined weight of the carrier mixture and monomer mixture; ii) the weight of the monomer mixture is at least 200 times the weight of the seed particles and when polymerized is sufficient to provide the polymer polyol with at least 30 weight-percent dispersed polymer particles and iii) the amount of the solvent is 6 to 20% of the combined weight of the carrier mixture and the monomer mixture i) the amount of seed dispersion is sufficient to provide 0.025 to 0.35 weight percent seed particles and 0.1 to 0.75 weight percent of unsaturated macromer based on the combined weight of the carrier mixture and monomer mixture; ii) the weight of the monomer mixture is at least 200 times the weight of the seed particles and when polymerized is sufficient to provide the polymer polyol with at least 30 weight-percent dispersed polymer particles and iii) the amount of the solvent is 6 to 20% of the combined weight of the carrier mixture and the monomer mixture c) polymerizing the dispersed monomer mixture to form polymer particles dispersed in the carrier mixture and then d) removing the solvent from the carrier mixture to form a polymer polyol product containing at least 30 weight-percent dispersed polymer particles, wherein the dispersed polymer particles have a particle size of 1 to 3 μm and a particle size span of less than 1.25