Poly(ethylene glycol)-based nanoparticles have received significant attention in the field of biomedicine. When they are copolymerized with pH- or temperature-sensitive comonomers, their small size allows them to respond very quickly to changes in the environment, including changes in the pH, ionic strength, and temperature. In addition, the high surface-to-volume ratio makes them highly functionalized. In this work, nanoparticles composed of temperature-sensitive poly(N-isopropylacrylamide), poly(ethylene glycol) 400 dimethacrylate, and poly(ethylene glycol) 1000 methacrylate were prepared by a thermally initiated, free-radical dispersion polymerization method. The temperature-responsive behavior of the hydrogel nanoparticles was characterized by the study of their particle size with photon correlation spectroscopy. The size of the nanoparticles varied from 200 to 1100 nm and was a strong function of the temperature of the system, from 5 to 40degreesC. The thermal, structural, and morphological characteristics were also investigated.