This study investigates whether empathic distress and empathic concern at 20 months predict aggression at 20 and 30 months, while considering the moderating role of sex. The research involved 133 toddlers (69 boys) at 20 months, with 119 (62 boys) followed up at 30 months. Empathic behavior was observed during a distress simulation task, and physical aggression was assessed via maternal reports using the Physical Aggression Scale for Early Childhood. Linear regression analyses revealed sex differences in the associations between empathic distress and concern and physical aggression. Specifically, more empathic distress and concern were associated with less physical aggression in girls, but not in boys. The findings indicate that the prediction of physical aggression by empathic distress was more robust over time than for empathic concern. The study highlights the complex relationship between empathy, aggression, and sex from a developmental perspective, suggesting that empathy may play a crucial role in reducing aggression, particularly in girls. The results emphasize the importance of considering sex differences in empathy and aggression research, as well as the potential for interventions targeting empathy to reduce aggression in toddlers. The study also notes that while empathy and aggression are generally negatively associated, this relationship may be influenced by factors such as the type of empathic behavior and sex differences. The study contributes to the understanding of the developmental trajectory of empathy and aggression, and the role of sex in shaping these relationships.This study investigates whether empathic distress and empathic concern at 20 months predict aggression at 20 and 30 months, while considering the moderating role of sex. The research involved 133 toddlers (69 boys) at 20 months, with 119 (62 boys) followed up at 30 months. Empathic behavior was observed during a distress simulation task, and physical aggression was assessed via maternal reports using the Physical Aggression Scale for Early Childhood. Linear regression analyses revealed sex differences in the associations between empathic distress and concern and physical aggression. Specifically, more empathic distress and concern were associated with less physical aggression in girls, but not in boys. The findings indicate that the prediction of physical aggression by empathic distress was more robust over time than for empathic concern. The study highlights the complex relationship between empathy, aggression, and sex from a developmental perspective, suggesting that empathy may play a crucial role in reducing aggression, particularly in girls. The results emphasize the importance of considering sex differences in empathy and aggression research, as well as the potential for interventions targeting empathy to reduce aggression in toddlers. The study also notes that while empathy and aggression are generally negatively associated, this relationship may be influenced by factors such as the type of empathic behavior and sex differences. The study contributes to the understanding of the developmental trajectory of empathy and aggression, and the role of sex in shaping these relationships.