Seven hundred thirty-one income-eligible families in 3 geographical regions who were enrolled in a national food supplement program were screened and randomized to a brief family intervention. At child ages 2 and 3, the intervention group caregivers were offered the Family Check-Up and linked parenting support services. Latent growth models on caregiver reports at child ages 2, 3, and 4 revealed decreased behavior problems when compared with the control group. Intervention effects occurred predominantly among families reporting high levels of problem behavior at child age 2. Families in the intervention condition improved on direct observation measures of caregivers’ positive behavior support at child ages 2 and 3; improvements in positive behavior support mediated improvements in children’s early problem behavior.