The Parent-Child Relationally Informed Early Intervention was developed from the clinical application of attachment theory. The premise is to expand the responsiveness of parents and clinicians working with children who have developmental delays. It was evaluated via four single case studies (children aged 18-months to 6 years) using an AB(A) design including baseline assessment and evaluation across 12-week therapy blocks. Parents provided session-by-session feedback. A battery of measures was used pre- and post-treatment. An independent evaluator assessed fidelity. Functional gains were reported. Parental stress, parent/child relational stress and difficult child rating decreased. Parental sense of competence increased; there were slight gains in well-being measures. Results suggest PCRI-EI is a promising intervention for engaging parents in therapy and achieving positive outcomes.