This study investigated the effect of a mindfulness-based intervention on pathological symptoms in boys with externalised disorders. A total of 24 elementary school students with externalising disorders, diagnosed by completing the Child Behavior Checklist (CBCL), were randomly assigned to a mindfulness-based intervention for two months. One session per week was offered and each session lasted one hour (n = 12) or as a wait-list control group (n = 12). Data were analysed via a multivariate analysis of covariance (MANCOVA) test. Students from the intervention group showed greater reductions in rule-breaking behaviours than those in the wait-list control group. Also, there was clear superiority of the intervention relative to the wait-list control group for reducing both aggression and rule-breaking behaviours (effect size differences were d = -2.52 and d = -1.88 respectively). These positive results on outcome measures provide initial evidence for a mindfulness-based intervention as a treatment option for boys with externalising disorders.