Purpose. A blind, severely intellectually impaired boy aged 17 with Down syndrome and persistent serious challenging behavior received attachment-based behavior modification treatment. The aim was to study the effect of the treatment and the development of the therapeutic attachment relationship. Method. In a single-case study, attachment therapy sessions alternated with control sessions. Treatment started with attachment therapy (phase 1), followed by behavior modification (phase 2). The instruments used were: Residential observation lists for challenging behavior, video analyses of attachment behavior in therapy sessions and physiological indicators of affect regulation measuring the pre-ejection period (PEP) and respiratory sinus arrhythmia (RSA) as indices of cardiac sympatho-vagal activity. Results. The client exhibited less frequent and less intensely challenging behavior. The data indicated more appropriate replacement behavior and less PEP arousal during the behavior modification treatment given by the attachment therapist compared to the control therapist who used the same protocol. The client showed more active and longer-lasting attachment behavior, especially proximity seeking, towards the attachment therapist than towards the control therapist. Conclusions. Attachment-based psychotherapy proved successful in eliciting attachment behavior in a severely intellectually disabled, socially deprived, behaviorally and affectively dysregulated adolescent. The resulting relationship proved to be a therapeutic platform conducive to behavior change.