OBJECTIVE: To determine whether a parenting programme, offered universally in primary care, can prevent behavioural problems in children and improve parenting and maternal mental health. DESIGN: Cluster randomised trial. SETTING: 40 primary care nursing centres (clusters) in Victoria, Australia. PARTICIPANTS: 733 English speaking mothers of 8 month old children sequentially recruited from well child appointments; 656 retained at 24 months. INTERVENTION: Structured three session programme at age 8-15 months, co-led by well child providers and a parenting expert. The programme covered normal development and behaviour, strategies to increase desired behaviour, and strategies to reduce unwanted behaviour. MAIN OUTCOME MEASURES: Maternal report of child externalising behaviour (child behavior checklist 1(1/2)-5 year old), parenting (parent behavior checklist), and maternal mental health (depression anxiety stress scales) at 18 and 24 months. RESULTS: At 18 months, child behaviour and parenting scores were similar in the two groups. At 24 months, externalising scores in the intervention and control groups were similar (mean 11.9 (SD 7.2) v 12.9 (7.4)); however, on the parent behavior checklist subscale scores, intervention group parents were less likely to report harsh/abusive parenting (mean 38.9 (SD 7.7) v 40.5 (8.8); adjusted mean difference -1.83, 95% confidence interval -3.12 to -0.55) and unreasonable expectations of child development (40.9 (9.9) v 42.7 (9.6); -2.18, -3.74 to -0.62). Mean scores for nurturing parenting and maternal mental health were similar in the two groups at both times. CONCLUSIONS: A universal parenting programme resulted in modest improvement in parenting factors that predict behavioural problems in children but did not reduce externalising behavioural problems or affect maternal mental health at 2 years. Trial registration ISRCTN 77531789.