Interventions to develop functional capacity for children with developmental difficulties have tended to be therapist driven. Such approaches can present difficulties for parents who describe increased stress and feelings of incompetence in a paradigm that can feel imposed. Recent findings have questioned the efficacy of the approach, given a deeper understanding of the impact of social and relational influences on child development. This questioning has prompted the development of programmes where the parent-child relationship is considered a fundamental component of early childhood intervention. The Parent-Child Relationally Informed Early Intervention (PCRI-EI) is such a programme. The current study presents a quasi-experimental assessment of the efficacy of PCRI-EI in achieving functional gains in a sample of 56 children with differing diagnoses presenting to a community early childhood development service. Significant and marked (i.e. large effect size) increases in functional capacity were observed across time. The gains did not differ by diagnosis and parental feedback indicated the improvements were generalised across social, community and educational settings.