This article addresses whether young children's play-partner choices are stable over time and how these choices influence behavior. Sixty-one children (28 boys and 33 girls; mean age = 53 months) were observed over 6 months, and type of play behavior and sex of play partners were recorded. Children's partner preferences were highly sex differentiated and stable over time, especially when larger aggregates of data were used. Two types of consequences were identified: a binary effect that influenced differences between the sexes and a social dosage effect that influenced variations within the sexes. The binary effect reflected a pattern in which the more both girls and boys played with same-sex partners, the more their behavior became sex differentiated. The social dosage effect reflected a pattern in which variations in levels of same-sex play in the fall contributed significantly to variations in the spring above initial levels of the target behaviors.