This study investigates parental factors as predictors of the outcome of an internet-based and telephone-assisted parent training intervention targeting child disruptive behavior, when it was implemented nationwide in Finland. 2,900 families with children who met screening criteria at their 4-year-old health checkup received the 11-week intervention. Potential predictors included parents’ age and educational attainment, family structure, enrollment year, parenting skills, and parental mental health. The outcome was the Child Behavior Checklist 1.5-5 externalizing score, collected at baseline, 6-, 12-, and 24-month follow-ups. Hierarchical linear models were used to examine changes in the outcome over time. At 6 months, parents of children whose mothers had a college or university degree reported smaller reductions in child externalizing problems compared to those with lower maternal education (β= 0.87, 95% CI [0.28, 1.45], p= .004). At 24 months, greater reductions in externalizing problems were observed in children whose parents had a high parental over-reactivity score at baseline (β= -1.23, 95% CI [-1.97, -0.49], p= .001), as well as a medium or high baseline parental Depression, Anxiety and Stress Scale total score (Medium: β= -1.01, 95% CI [-1.65, -0.36], p= .002; High: β= -1.05, 95% CI [-1.82, -0.28], p= .007). There were greater changes in the outcome over time among disadvantaged populations, particularly families with lower education, poorer parenting skills, or more severe parental mental health problems. Future research is needed to examine the consistency of these effects across diverse sociodemographic groups and settings.