Objective: To determine whether an internet-based and telephone-assisted parent training intervention, which used whole population screening and which comprehensively targeted symptomatic 4-year-old children, was effective 24 months after the start of treatment. No long-term follow-up studies using a randomized controlled trial (RCT) existed on this subject.

Method: Of the 4,656 children 4 years old who were screened in Southwest Finland, 730 met the criteria for high-level disruptive behavioral problems, and 464 parents agreed to be randomized to the 11-week Strongest Families Smart Website (SFSW) intervention (n = 232) or an educational control (EC) (n = 232). After 24 months, 163 SFSW parents (70%) and 165 EC parents (71%) were still participating.

Results: When we compared the results at baseline and 24 months, the primary outcome of the Child Behavior Checklist (CBCL) externalizing score showed significantly higher improvements in the SFSW group (effect size 0.22; p < 0.001). This group also showed greater improvements in the secondary outcomes: the CBCL total and internalizing scales, 5 of the 7 CBCL symptom domains, 3 of the 5 DSM subscores, and self-reported parenting skills. Fewer SFSW children (17.5%) than EC children (28.0%) had been referred to child mental health services between baseline and 24 months (odds ratio = 1.8; 95% confidence interval = 1.1−3.1).

Conclusion: The SFSW internet-based and telephone-assisted parental training program was effective 24 months after initiation, underlining the value of identifying children at risk in the community early and providing evidence-based parent training for a large number of families.