This study examined the reciprocal associations between sleep (duration and quality) and aggressive behavior in toddlers. The sample consisted of 82 children (43 boys and 39 girls) and their parents who completed two assessments, when children were 2 and 3 years old. At both time points, children wore an actigraph for 3 consecutive days to assess their sleep patterns, and both their parents reported on their child's aggressive behavior. The results indicated negative associations between sleep quality at age 2 and both parents' evaluations of aggressive behavior at age 3; in contrast, the relations between aggression at age 2 and sleep (duration or quality) at age 3 were small and nonsignificant. In line with studies of older children and adolescents, these results suggest that sleep difficulties are more likely to favor the emergence of aggressive behavior among young children than the converse.