Children's emotional reactivity may interact with their regulatory behaviors to contribute to internalizing problems and social functioning even early in development. Ninety-one preschool children participated in a longitudinal project examining children's reactivity and regulatory behaviors as predictors of internalizing problems, and positive and negative social behavior in the classroom. Children who paired negative emotion expression with disengagement during a laboratory task showed higher levels of internalizing problems and more negative social behavior in the classroom 6 months later. Positive emotion expression paired with engagement during a laboratory task predicted more positive social behavior in the classroom 6 months later. Physiological reactivity and regulation also predicted children's social behavior in the classroom. Findings suggest that preschool children with maladaptive reactivity and regulatory patterns may be at greater risk for internalizing problems even in early childhood.