Differential emotions theory (DET) ("The face of emotion." Appleton-Century-Crofts: East Norwalk, CT, 1971) posits that the smile functions in part to communicate and/or reflect social affiliation and plays an important role in children's social development. While children's positive emotion expressions have received attention from peer relations researchers in observational studies and within correlational designs, there is almost no experimental evidence for the impact of the smile. Building on existing studies, the present study examined DET predictions within an experimental design. More specifically, we examined the impact smiles have on lower-income preschool children's nominations for preferred playmates. Both boys and girls tended to nominate unfamiliar playmates who smiled. Additionally, some evidence suggested that preference for smiling playmates seemed more critical to girls' than boys' social functioning. Unlike among boys, teachers rated girls who exhibited a greater preference for smiling playmates as higher in social competence. (Contains 1 table.)