BACKGROUND: Child maltreatment is a significant public health problem. Group Family Nurse Partnership (gFNP) is a new intervention for young, expectant mothers implemented successfully in pilot studies. This study was designed to determine the effectiveness and cost-effectiveness of gFNP in reducing risk factors for maltreatment with a potentially vulnerable population. METHODS: A multi-site, randomized controlled, parallel-arm trial and prospective economic evaluation was conducted, with allocation via remote randomization (minimization by site, maternal age group) to gFNP or usual care. Participants were expectant mothers aged below 20 years with at least one live birth, or aged 20-24 years with no live births and with low educational qualifications. Data from maternal interviews at baseline and when infants were 2, 6 and 12 months, and video-recording at 12 months, were collected by researchers blind to allocation. Cost information came from weekly logs completed by gFNP family nurses and other service delivery data reported by participants. Primary outcomes measured at 12 months were parenting attitudes (Adult-Adolescent Parenting Index, AAPI-2) and maternal sensitivity (CARE Index). The economic evaluation was conducted from a UK NHS and personal social services perspective with cost-effectiveness expressed in terms of incremental cost per quality-adjusted life year (QALY) gained. The main analyses were intention-to-treat with additional complier average causal effects (CACE) analyses. RESULTS: Between August 2013 and September 2014, 492 names of potential participants were received of whom 319 were eligible and 166 agreed to take part, 99 randomly assigned to receive gFNP and 67 to usual care. There were no between-arm differences in AAPI-2 total (7 · 5/10 in both, SE 0.1), difference adjusted for baseline, site and maternal age group 0 · 06 (95% CI - 0 · 15 to 0 · 28, p = 0 · 59) or CARE Index (intervention 4 · 0 (SE 0 · 3); control 4 · 7 (SE 0 · 4); difference adjusted for site and maternal age group - 0 · 68 (95% CI - 1 · 62 to 0 · 16, p = 0 · 25) scores. The probability that gFNP is cost-effective based on the QALY measure did not exceed 3%. CONCLUSIONS: The trial did not support gFNP as a means of reducing the risk of child maltreatment in this population but slow recruitment adversely affected group size and consequently delivery of the intervention. TRIAL REGISTRATION: ISRCTN78814904 . Registered on 17 May 2013.