The present investigation was designed to examine systematically the effects of short-term crowding, involving close physical proximity, on children and adolescents. Fourth-, eighth-, and eleventh-grade children participated in groups of 4 under high or moderate spatial density conditions. During exposure to these conditions, physiological responses were measured, and following this experience children engaged in a cooperation-competition activity and provided self-reports related to their participation. Results demonstrated that crowding had physiological, behavioral and social consequences for the children studied. Across all grade levels, males in particular were more affected by short-term crowding, displaying the highest elevations in stress-related arousal. Following exposure to the crowding conditions, children were more competitive, despite the fact that they had all to gain from cooperating and nothing to lose. Furthermore, children reported having felt crowded, tense, annoyed, and uncomfortable as a function of the close physical proximity to others. Implications of the immediate and residual effects of crowding on the behavior and functioning of children are discussed.