Background: One of the most important facts that determine human development is the socioeconomic status (SES), specifically, low SES and poverty. Likewise, family environment is essential in the development of children and a potential mediator or moderator of the effect of other social conditions in children. Objective: To analyze the role of parenting stress as a variable that mediates the relationship between SES and both externalized and internalized behaviors in preschool children. Subjects and Method: Descriptive secondary base study based on the Longitudinal Survey of Chilean First Infancy that selected a stratified sample, representative by clusters, of 9.996 children from 3 to 5 years old and their caregivers, that completed a battery of instruments for measuring SES variables, parenting stress and externalized and internalized behaviors. The analysis used a linear model with least square estimate. As hypothesis test, the Dm (an adaptation of the F-test for multiple account assignment) was used. Results: The mediation model of parenting stress in the relationship between SES and both externalized and internalized behaviors was confirmed for the latter; regarding externalized behaviors, a model of moderation was observed, being the stress influence lower on the low SES. Conclusions: Parental stress showed a clear relationship with the presence of externalized and internalized behaviors, stronger than the SES. The relationship between SES and parenting stress is very important to understand the processes that affect children's development.