Background: Tobacco smoke pollution (TSP) has major negative effects on infant health. Our objectives were to determine the effectiveness of a brief primary care intervention directed at parents who smoke in reducing babies' TSP exposure, and to establish variables related to greater exposure. Method: A multicentre, open, cluster-randomised clinical trial in Catalonia. The 83 participating primary health paediatric teams of the Catalan Health Service recruited 1101 babies whose parents were smokers. The intervention group (IG) received a brief TSP intervention; the control group (CG) received the usual care. Outcomes were measured by parents' reported strategies to avoid TSP exposure. Baseline clinical data and characteristics of each baby's TSP exposure were collected, along with infant hair samples and parents' tobacco use and related attitudes/behaviours. At 3-month and 6-month follow-up, behavioural changes to avoid TSP exposure were recorded; the association between reported parental behaviours and nicotine concentration in infant hair samples was tested in a random sample of 253 babies at baseline and 6 months. Results: During follow-up, TSP-avoidance strategies improved more in the IG than in the CG: 35.4% and 26.9% ( p=0.006) at home, and 62.2% and 53.1% in cars (p=0.008). Logistic regression showed adjusted ORs for appropriate measures in the IG versus CG of 1.59 (95% CI 1.21 to 2.09) at home and 1.30 (95% CI 0.97 to 1.75) in cars. Hair samples showed that 78.7% of the babies tested were exposed. Reduced nicotine concentration was associated with improved implementation of effective strategies reported by parents at home ( p=0.029) and in cars ( p=0.014). Conclusions: The intervention produced behavioural changes to avoid TSP exposure in babies. The proportion of babies with nicotine (>=1ng/mg) in hair samples at baseline is a concern.