BackgroundPsychopathology in women after childbirth represents a significant risk factor for parenting and infant mental health. Regarding child development, these infants are at increased risk for developing unfavorable attachment strategies to their mothers and for subsequent behavioral, emotional and cognitive impairments throughout childhood. To date, the specific efficacy of an early attachment-based parenting group intervention under standard clinical outpatient conditions, and the moderators and mediators that promote attachment security in infants of mentally ill mothers, have been poorly evaluated.Methods/DesignThis randomized controlled clinical trial tests whether promoting attachment security in infancy with the Circle of Security (COS) Intervention will result in a higher rate of securely attached children compared to treatment as usual (TAU). Furthermore, we will determine whether the distributions of securely attached children are moderated or mediated by variations in maternal sensitivity, mentalizing, attachment representations, and psychopathology obtained at baseline and at follow-up. We plan to recruit 80 mother-infant dyads when infants are aged 4-9 months with 40 dyads being randomized to each treatment arm. Infants and mothers will be reassessed when the children are 16-18 months of age. Methodological aspects of the study are systematic recruitment and randomization, explicit inclusion and exclusion criteria, research assessors and coders blinded to treatment allocation, advanced statistical analysis, manualized treatment protocols and assessments of treatment adherence and integrity.DiscussionThe aim of this clinical trial is to determine whether there are specific effects of an attachment-based intervention that promotes attachment security in infants. Additionally, we anticipate being able to utilize data on maternal and child outcome measures to obtain preliminary indications about potential moderators of the intervention and inform hypotheses about which intervention may be most suitable when offered in a clinical psychiatric outpatient context.Trial registrationCurrent Controlled Trials ISRCTN88988596