Despite the importance of understanding sympathy and prosocial behaviors, research on the development of these tendencies in adolescence remains relatively sparse. In the present study, we examined age trends and bidirectional longitudinal relations in sympathy and prosocial behaviors across early to middle adolescents. Participants were 500 12-year-olds at Time 1 (52% girls, 70% European American) who completed measures of sympathy and prosocial behaviors at 5 different time points, each approximately 1 year apart. Results showed significant bidirectional relations between sympathy and prosocial behaviors across all time points, and an initial decrease of prosocial behaviors followed by an increase into middle adolescence. The implications for prosocial developmental theories and research are discussed.