Objective: To investigate whether early developmental intervention (EDI) can positively affect the trajectories of cognitive development among children from low-resource families. Methods: Longitudinal analyses were conducted of data from 293 children in the Brain Research to Ameliorate Impaired Neurodevelopment Home-based Intervention Trial, a randomized controlled trial of a home-based EDI program, to examine trajectories of Bayley Scales of Infant Development-Second Edition Mental Development Index (MDI) scores from 12 to 36 months of age among young children from high-and low-resource families in 3 low-to middle-resource countries. Results: A 3-way interaction among family resources, intervention group, and age was statistically significant after controlling for maternal, child, and birth characteristics (Wald χ2(1) = 9.41, P =.002). Among children of families with high resources, both the intervention and control groups had significant increases in MDI scores over time (P <.001 and P =.002, respectively), and 36-month MDI scores for these 2 groups did not differ significantly (P =.602). However, in families with low resources, the EDI group displayed greater improvement, resulting in significantly higher 36-month MDI scores than the control group (P <.001). In addition, the 36-month MDI scores for children in families with low resources receiving EDI did not differ significantly from children from high-resource families in either the EDI (P =.509) or control (P =.882) groups. Conclusions: A home-based EDI during the first 3 years of life can substantially decrease the developmental gap between children from families with lower versus higher resources, even among children in low-to middle-resource countries.