OBJECTIVE: To determine whether a structured programme of parent anticipatory guidance 'Toddlers Without Tears,' delivered in universal primary care, can prevent preschool child behaviour problems. DESIGN: Cluster randomised controlled trial. SETTING/PARTICIPANTS: 40 primary care nursing centres (clusters) in metropolitan Melbourne, Australia. 733 English-speaking mothers of 6- to 7-month-old infants consecutively recruited from well-child appointments; 589 (80%) retained at age 3 years. INTERVENTION: Parenting programme from age 8 to 15 months, with two group sessions co-led by well-child providers and a parenting expert. The programme addressed normal behaviour development and offered strategies to increase desired and reduce unwanted behaviour. While 93% of intervention parents received at least some of the programme, only 49% completed all components. Control Usual primary care. MAIN OUTCOME MEASURES: Maternal report of child externalising behaviour (Child Behaviour Checklist), parenting (Parent Behaviour Checklist) and maternal mental health (Depression Anxiety Stress Scales) when children were aged 3 years. RESULTS: Behaviour scores in the intervention and control groups were similar (mean (SD)) 11.4 (7.1) versus 12.4 (7.6); adjusted mean difference -0.8, 95% CI -2.2 to 0.6, p=0.26). On the parenting subscale scores, intervention parents reported fewer unreasonable expectations of child development (37.3 (10.9) vs 39.9 (10.2), adjusted mean difference -3.1, 95% CI -4.9 to -1.4, p=0.001). The mean scores for harsh/abusive and nurturing parenting, and maternal mental health, were similar between the two groups. CONCLUSIONS: A brief universal parenting programme in primary care is insufficient to prevent development of preschool externalising problems. A new population trial targeting more intensive prevention to distressed parents with toddler behaviour problems is now under way, aiming to prevent externalising and internalising problems before school entry. TRIAL REGISTRATION NUMBER: ISRCTN77531789.