|Accumulating evidence suggests proactive and reactive aggression are uniquely related to a variety of negative outcomes. Children with higher levels of reactive, not proactive, aggression score lower on measures of social preference and score higher on measures of internalizing symptoms (Card & Little, 2006). There is also evidence that aggressive children struggle more with loneliness (Crick & Grotpeter, 1995). The current study sought to examine whether proactive and reactive aggression uniquely predict self-reported loneliness and whether social preference mediates these relations. A total of 345 children ages 6-10 were recruited from local elementary schools. Data were collected in the Fall and Spring of a single academic year. Structural equation modeling was used to assess the associations between proactive and reactive aggression and loneliness, and bias corrected bootstrapping was used to assess the significance of social preference as a mediator. Results indicated that models significantly differed by gender and timepoint. Results from a single-time point model indicated that for boys, higher levels of reactive aggression were associated with greater loneliness, and this association was mediated by social preference. For girls, higher levels of reactive aggression were associated with lower social preference and higher loneliness at the trend level. Contrary to predictions, social preference mediated the association between proactive aggression and loneliness, but not reactive aggression. When examining the model across timepoints and controlling for initial levels of social preference and loneliness, few significant paths remained. For boys, neither proactive nor reactive aggression were associated with social preference or loneliness. For girls, reactive aggression was negatively associated with social preference at the end of the school year. In the multi-time point models, social preference was not a mediator of the aggression-loneliness association. Thus, results indicate that social preference mediates the association between aggression and loneliness; however, this varies based on type of aggression, gender, and whether associations are examined concurrently versus across time. Future research should further examine the complicated relationships of aggression, social preference, and loneliness.