The authors used a population-based birth cohort of 3,253 children (52% males) born in Brisbane, Australia, between 1981 and 1984 to prospectively examine whether maternal smoking during pregnancy was associated with offspring overweight and obesity. The authors compared mean body mass indexes (weight (kg)/height (m)2) and levels of overweight and obesity at age 14 years among offspring by patterns of maternal smoking (never smoked, smoked before and/or after pregnancy but not during pregnancy, or smoked during pregnancy). Adolescent body mass index and prevalences of overweight and obesity were greater in offspring whose mothers had smoked during pregnancy than in those whose mothers had never smoked. Body mass index and levels of overweight and obesity among adolescent offspring whose mothers stopped smoking during pregnancy but smoked at other times in the child's life were similar to those among offspring whose mothers had never smoked. These results were independent of a range of potentially confounding factors and suggest a direct effect of maternal smoking during pregnancy on adolescent overweight and obesity. They provide yet another incentive for pregnant women to be persuaded not to smoke and for young women to be encouraged to never take up smoking.