Despite potential sex differences in base rates, predictors, and maintaining processes for children's externalizing behaviors, little prospective research has examined sex differences in the relations between concurrent, proximal family risk factors and children's externalizing behaviors. The current study examined the relations among maternal depressive symptoms, maternal parenting behaviors (i.e., negativity and low warmth), and child externalizing symptoms at 24 months and first grade in a community-based sample of 1,364 children enrolled in the National Institute of Child Health and Human Development Study of Early Child Care and Youth Development. Structural equation modeling revealed that maternal depression and negative parental behaviors were associated with concurrent externalizing behaviors, though maternal depression may be differentially linked to boys' and girls' externalizing problems. The relation between depression and boys' externalizing symptoms was more pronounced at 24 months, and over time, the relation between maternal depression and boys' externalizing symptoms decreased in magnitude, whereas this relation increased among girls.