Background: The positive effects of early developmental intervention (EDI) on early child development have beenreported in numerous controlled trials in a variety of countries. An important aspect to determining the efficacy ofEDI is the degree to which dosage is linked to outcomes. However, few studies of EDI have conducted such analyses.This observational cohort study examined the association between treatment dose and children's development whenEDI was implemented in three low and low-middle income countries as well as demographic and child health factorsassociated with treatment dose.Methods: Infants (78 males, 67 females) born in rural communities in India, Pakistan, and Zambia received aparent-implemented EDI delivered through biweekly home visits by trainers during the first 36 months of life.Outcome was measured at age 36 months with the Mental (MDI) and Psychomotor (PDI) Development Indices of theBayley Scales of Infant Development-II. Treatment dose was measured by number of home visits completed andparent-reported implementation of assigned developmental stimulation activities between visits. Sociodemographic,prenatal, perinatal, and child health variables were measures as correlates.Results: Average home visits dose exceeded 91% and mothers engaged the children in activities on average62.5% of days. Higher home visits dose was significantly associated with higher MDI (mean for dose quintiles 1-2combined = 97.8, quintiles 3-5 combined = 103.4, p = 0.0017). Higher treatment dose was also generally associatedwith greater mean PDI, but the relationships were non-linear. Location, sociodemographic, and child health variableswere associated with treatment dose.Conclusions: Receiving a higher dose of EDI during the first 36 months of life is generally associated with betterdevelopmental outcomes. The higher benefit appears when receiving ≥ 91% of biweekly home visits and programactivities on ≥ 67% of days over 3 years. It is important to ensure that EDI is implemented with a sufficiently high doseto achieve desired effect. To this end groups at risk for receiving lower dose can be identified and may require specialattention to ensure adequate effect.