Composite clay/polymer nanoparticle dispersions have been studied for many different polymers but remained elusive for polyolefins even though polyolefins are an important class of polymers and are also industrially applied in the form of aqueous dispersions. Despite the hydrophobic and apolar nature of polyethylene, aqueous nanocomposite dispersions are formed via heterogeneous free-radical polymerization in the presence of the synthetic smectite clay Laponite-RD at relatively mild conditions (250 atm, 85 degrees C). The clay nanoplatelets effectively contribute to colloidal stability of the polyolefin particles formed and strongly enhance particle formation during the aqueous polymerization. Evolution of the polymer-clay interaction at an early stage of particle formation is instrumental, and initiator-derived polar end groups are decisively involved. The clay enhances polymerization yields and promotes the formation of smaller particles. Stable dispersions with polymer contents of up to 24% were obtained without the need of additional organic surfactants.