Introduction: Behavioural techniques reduce infant sleep problems and associated maternal depression, yet their safety remains in doubt due to theoretical concerns about harming child-parent attachment and children’s subsequent mental health. We aimed to determine whether, at age 6 years, a behavioural sleep program delivered in infancy affects ‘attachment-related’ outcomes including child mental health, psychosocial functioning, chronic stress regulation, the child-parent relationship and parenting styles. Methods: Design and Setting: Five-year follow-up of a population based cluster-randomized controlled trial, conducted across well-child centres in Melbourne, Australia. Participant allocation was concealed and researchers (but not parents) blinded to group allocation. Participants: Three hundred and twenty-six children (173 intervention) with parent-reported sleep problems at age 7 months. Intervention: Behavioural strategies delivered over 1–3 individual nurse consultations at infant age 8–10 months, versus usual care. Main Outcome Measures: Child mental health (primary outcomes: SDQ Emotion and Conduct subscales), psychosocial functioning (PedsQL), stress regulation (diurnal salivary cortisol), child-parent relationship (CPR-Scale, disinhibited attachment, global rating), parenting styles (authoritarian/permissive/disengaged versus authoritative). Results: Two hundred and twenty-five families (69%) were retained to age 6 years. SDQ Emotion and Conduct subscale scores were similar between groups [adjusted mean difference (95% confidence interval) – 0.04 (-0.6 to 0.5) and 0.1 (-0.3 to 0.6), respectively] and there was little evidence of differences between trial arms for any other attachment related outcome. Conclusions: Behavioural sleep techniques appear to have no marked long-lasting effects (positive or negative) on child or maternal outcomes. Parents and health professionals can feel comfortable offering these strategies (previously been shown to cost-effective) to reduce the short-to-medium-term population burden of infant sleep problems and maternal depression.