Objective: Interventions to reduce health inequalities for young children and their mothers are important: involving peers is recommended, but evidence of value for this approach is limited. The authors aimed to examine the effect of an innovative tailored peer-mentoring programme, based on perceived needs, for first-time mothers in socio-economically deprived communities. Design: Randomised controlled trial; parallel qualitative study with purposive samples using semistructured interviews. Setting: Socio-economically disadvantaged areas, Belfast. Participants: Primigravidae, aged 16-30 years, without significant co-morbidity. Intervention: Peer-mentoring by a lay-worker fortnightly during pregnancy and monthly for the following year, tailored to participants' wishes (home visits/telephone contacts), additional to usual care. Main outcome measures: Infant psychomotor and mental development (Bayley Scales of Infant Development (BSID-II)) at 1 year, assessed by an observer blinded to group allocation. Mothers' health at 1 year postnatal (SF-36). Results: Of 534 women invited, 343(64%) participated; 85%, with their children, completed outcome assessments (140 of 172 intervention; 152 of 171 controls). Intervention and control groups did not differ in BSID-II psychomotor (mean difference 1.64, 95% CI -0.94 to 4.21) or mental (-0.81, -2.78 to 1.16) scores, nor SF-36 physical functioning (-5.4, -11.6 to 0.7) or mental health (-1.8, -6.1 to 2.6). Women valued advice given in context of personal experience of child-rearing. Mentors gained health-related knowledge, personal skills and new employment opportunities. Conclusions: Despite possible longer-term social advantage, this peer-mentoring programme showed no benefit for infant development or maternal health at 1 year. Further rigorous evaluation of important outcomes of complex interventions promoting health for children in socially disadvantaged communities is warranted. Trial registration no: ISRCTN 55055030.