This study examined the efficacy of the attachment-based Circle of Security 20-week intervention in improving child behavioural and emotional functioning. Participants were 83 parents of children (1–7 years) referred to a clinical service with concerns about their young children’s behaviour. Parents (and teachers, when available) completed questionnaires assessing child protective factors, behavioural concerns, internalizing and externalizing problems, prior to and immediately after the intervention. The following were considered as potential moderators: child gender and age, parent representations, reflective functioning, child attachment indices and severity of presenting problems, prior to treatment. Results showed significant improvement for parent ratings of child protective factors, behavioural concerns, internalizing and externalizing symptoms, all ps < .05, and children with more severe problems showed most improvement. Teachers also reported improvements, but change was significant only for externalizing problems (p = .030). Findings suggest Circle of Security is effective in improving child behavioural and emotional functioning in clinically referred children aged 1–7 years.