ABSTRACT: BACKGROUND: Several studies suggest that the number of risk factors rather than their nature is key to mental health disorders in childhood.Method and DesignThe objective of this multicentre randomized controlled parallel trial (PROBE methodology) is to assess the impact in a multi-risk French urban sample of a home-visiting program targeting child mental health and its major determinants. This paper describes the protocol of this study. In the study, pregnant women were eligible if they were: living in the intervention area; able to speak French, less than 26 years old; having their first child; less than 27 weeks of amenorrhea; and if at least one of the following criteria were true: less than twelve years of education, intending to bring up their child without the presence of the child's father, and 3) low income. Participants were randomized into either the intervention or the control group. All had access to usual care in mother-child centres and community mental health services free of charge in every neighbourhood. Psychologists conducted all home visits, which were planned on a weekly basis from the 7th month of pregnancy and progressively decreasing in frequency until the child's second birthday. Principle outcome measures included child mental health at 24 months and two major mediating variables for infant mental health: postnatal maternal depression and the quality of the caring environment. A total of 440 families were recruited, of which a subsample of 120 families received specific attachment and caregiver behaviour assessment. Assessment was conducted by an independent assessment team during home visits and, for the attachment study, in a specifically created Attachment Assessment laboratory. DISCUSSION: The CAPEDP study is the first large-scale randomised, controlled infant mental health promotion programme to take place in France. A major specificity of the program was that all home visits were conducted by specifically trained, supervised psychologists rather than nurses. Significant challenges included designing a mental health promotion programme targeting vulnerable families within one of the most generous but little assessed health and social care systems in the Western World.Trial registrationCurrent Clinical trial number is NCT00392847.