Theory of mind is one of the most important cognitive factors in social information-processing, and deficits in theory of mind have been linked to aggressive behavior in childhood. The present longitudinal study investigated reciprocal links between theory of mind and two forms of aggression--physical and relational--in middle childhood with three data waves over 3 years. Theory of mind was assessed by participants' responses to cartoons, and physical and relational aggression were assessed through teacher reports in a community sample of 1657 children (mean age at Time 1: 8 years). Structural equation modeling analyses showed that theory of mind was a negative predictor of subsequent physical and relational aggression, both from Time 1 to Time 2 as well as from Time 2 to Time 3. Moreover, relational aggression was a negative predictor of theory of mind from Time 1 to Time 2. There were no significant gender or age differences in the tested pathways. The results suggest that reciprocal and negative longitudinal relations exist between children's theory of mind and aggressive behavior. Our study extends current knowledge about the development of such relations across middle childhood.