Background  Screen time, defined as time spent watching television, DVDs, or videos or playing computer or video games, has been related to serious health consequences in children, such as impaired language acquisition, violent behaviour, tobacco smoking and obesity. Our aim was to determine if a simple intervention aimed at preschool‐aged children, applied at the health maintenance visits, in the primary care setting, would be effective in reducing screen time. Methods  We used a two group randomized controlled trial design. Two‐ to 6‐year‐old children and their parents were randomly assigned to receive an intervention to reduce their screen time, BMI and parental report of aggressive behaviour. At the end of the intervention we made home visits at 2, 6 and 9 months and the parents completed questionnaire. Results  Parents in the intervention group reported less screen time and less aggressive behaviour than those in the control group but there were no differences in BMI z scores. Conclusions  This study shows that a preschool‐based intervention can lead to reductions in young children's television/video viewing.