Objective—Existing research suggests that there is a relation between academic/cognitive deficits and externalizing behavior in young children, but the direction of this relation is unclear. The present study tested competing models of the relation between academic/cognitive functioning and behavior problems during early childhood Method—Participants were 221 children (120 boys, 101 girls) who participated in a longitudinal study from age 3 to 6 Results—A reciprocal relation (Model 3) was observed only between inattention and academic achievement; this relation remained controlling for SES and family stress. The relation between inattention and cognitive ability was consistent with Model 1 (cognitive skills predicting later inattention) with controls. For hyperactivity and aggression, there was some support for Model 2 (early behavior predicting later academic/cognitive ability), but this model was no longer supported when controlling for family functioning. Conclusion—These results suggest that the relation between academic achievement/cognitive ability and externalizing problems may be driven primarily by inattention. These results also suggest that this relation is evident early in development, highlighting the need for early assessment and intervention.