Childhood aggression has captured media attention over recent years. Aggression and violence have permeated schools and affected many communities. There are policies and programs in place for young adults, teens and children in some high schools and elementary schools, but not in preschools or daycare centers. However, intervention programs need to be introduced at the preschool level. There is also a scarcity of nursing research on aggression among preschoolers and successful early intervention anti-aggression programs.;This study evaluated the Second StepRTM anti-aggression program, utilized as a nursing intervention tool, among 41 preschool children aged 3, 4, and 5 years of age. The 3-month long research study was based on King's 1981 general systems theory, which is classified as an interaction model. The research design was a randomized pre-test post-test, 2-group (control and experimental) experimental one, to test the hypothesis that children's aggression scores would be lower and their prosocial scores would be higher after the intervention program. It was also hypothesized that boys would have higher aggression scores than girls and that there would be differences in post aggression scores in the treatment group.;Repeated measures ANOVA showed that there were no significant differences between the make-up of each group (p = .05). There was no statistically significant difference between pre- and post-test aggression scores (p = .14) or between genders (p = .13), with the exception that the preschool girls in both groups had slightly higher relational aggression scores than boys, pre- and post-test. The differences were statistically significant at p = <.05. The post-test relational aggression scores were not lower in either group. These findings are discussed in this paper.