The aim of the study was to assess whether the Aggression Replacement Training (ART) programme is effective in increasing social skills and decreasing problem behaviour. The sample consisted of 232 children (mean age 10.9 yrs, SD = 2.32), their parents and teachers. The study had a quasi-experimental design with intervention and control groups. Children were recruited from six schools and four social institutions from four regions in North-West Russia from 2010 to 2013. Social skills and externalizing behaviour were assessed with the "Social Skills Rating Scale" and analyzed by repeated measures ANOVA (GLM). In a pre and post-test assessment, the 30-hour ART programme was associated with a significant increase in social skills when assessed by children's self-reports. The most reliable effects of the intervention were demonstrated in the two age groups of 6-9 and 10-14 years old. When both pre and post-test were assessed by parents and teachers, children from both the intervention and control groups demonstrated more social skills and less problem behaviour. Overall results point to a significant improvement of social skills among children from the intervention groups, but an improvement in social skills and reduction of problem behaviour have also been indicated among children from the control group. Findings are discussed in view to possible diffusion of treatment from children participating in an intervention to children from control groups.