Objective: To assess the association between pregestational BMI and offspring's risk of overweight/obesity after accounting for the most important confounders, especially maternal smoking habit. Design: Prospective cohort study. Setting: The Seguimiento Universidad de Navarra (SUN) study is a prospective cohort of Spanish graduates with more than 22 000 participants nationwide. Recruitment started in 1999, and it is permanently open. Data on diet, lifestyle and clinical diagnoses are collected at baseline and every 2 years. Participants: Women from the SUN cohort who reported at least one pregnancy during follow-up (n 3496) were invited to this study. Among them, 1527 agreed to participate and completed an additional more detailed online questionnaire on their pregnancy history and their offspring's nutritional status. Results: After excluding 165 children, we analysed data of 2791 participants born to 1485 mothers and observed that each 5 kg/m(2) increase in pregestational BMI was associated with a 0 center dot 22 (95 % CI 0 center dot 15, 0 center dot 29) higher z-score in offspring's BMI and higher risk of overweight/obesity (multivariable-adjusted relative risk (RR) 1 center dot 57 (95 % CI 1 center dot 39, 1 center dot 77)) in childhood or adolescence. Furthermore, we observed stronger association in children born to smoker mothers (RR 1 center dot 91; 95 % CI 1 center dot 48, 2 center dot 46) than from non-smoker mothers (RR 1 center dot 51; 95 % CI 1 center dot 31, 1 center dot 73) (P-for interaction = 0 center dot 02). Conclusions: We found a synergistic interaction between pregestational BMI and maternal smoking habit on offspring's z-score of the BMI and in their risk of overweight/obesity. Although further research is needed to analyse dose-response relationships, these findings reinforce the importance of promoting healthy lifestyles in pregnant women in order to prevent childhood obesity.