OBJECTIVE: To evaluate the effectiveness of a parenting programme as a preventive intervention with parents of preschool children considered to be at risk of developing conduct disorder. DESIGN: Pragmatic randomised controlled trial using a block design with allocation by area. SETTING: Eleven Sure Start areas in north and mid-Wales. PARTICIPANTS: 153 parents from socially disadvantaged areas, with children aged 36-59 months at risk of conduct disorder defined by scoring over the clinical cut off on the Eyberg child behaviour inventory. Participants were randomised on a 2:1 basis, 104 to intervention and 49 to remaining on the wait listing (control). Twenty (13%) were lost to follow-up six months later, 18 from the intervention group. INTERVENTION: The Webster-Stratton Incredible Years basic parenting programme, a 12 week group based intervention. MAIN OUTCOME MEASURES: Problem behaviour in children and parenting skills assessed by self reports from parents and by direct observation in the home. Parents' self reported parenting competence, stress, and depression. Standardised and well validated instruments were used throughout. RESULTS: At follow-up, most of the measures of parenting and problem behaviour in children showed significant improvement in the intervention group. The intention to treat analysis for the primary outcome measure, the Eyberg child behaviour inventory, showed a mean difference between groups of 4.4 points (95% confidence interval 2.0 to 6.9, P<0.001) on the problem scale with an effect size of 0.63, and a mean difference of 25.1 (14.9 to 35.2, P<0.001) on the intensity scale with an effect size of 0.89. CONCLUSION: This community based study showed the effectiveness of an evidence based parenting intervention delivered with fidelity by regular Sure Start staff. It has influenced policy within Wales and provides lessons for England where, to date, Sure Start programmes have not been effective. TRIAL REGISTRATION: ISRCTN46984318.