Previous evidence has revealed an effect of environmental tobacco smoke (ETS) exposure on the development of childhood asthma. However, few studies have investigated the link between prenatal ETS exposure and adult-onset asthma. Therefore, in this study, we aim to determine if an association exists between adult-onset asthma and prenatal ETS exposure. Genetic predisposition to the development of asthma was also examined in conjunction with prenatal ETS exposure.This study was comprised of mothers and their offspring who were included in the Northern Finland Birth Cohort in 1966, a total of 12053 births. Data were collected from 7148 subjects, and 5491 met the inclusion criteria. The final study population consisted of 5200 subjects.Analysis was conducted on maternal smoking status in the last 3 months of pregnancy, and asthma status was conducted at 31 and 46 years of age (n = 5200). Questionnaire data were collected from subjects at 31 and 46 years of age regarding history of asthma or wheeze to determine incident asthma between 31 and 46 years. Information regarding maternal smoking status was gathered at clinical visits via questionnaire at 6- and 7-months’ gestation. Association between the development of asthma and maternal smoking (1 or more cigarette/pipe per day) was analyzed via Pearson’s χ2 test. Maternal smoking and associated changes in forced expiratory volume in 1 second (FEV1) and the FEV1 to forced vital capacity ratio were examined by using linear regression. Binary logistic regression was used to evaluate the relationship between maternal smoking status, adult-onset asthma, and single-nucleotide polymorphisms (SNPs). Linear regression controlled for covariates including maternal education and parental asthma as well as offspring smoking status, education, ponderal index, parity, and Apgar score at first minute.Maternal smoking during the last 3 months of pregnancy was associated with an increased risk for the development of asthma in offspring between 31 and 46 years of age, and reduced offspring FEV1 to forced vital capacity ratio at 31 years of age. This association was more prominent in individuals with haplotype rs11702779-AA and with the subgroup that reported previous wheeze. No association was found between maternal smoking during the last 3 months of pregnancy and offspring FEV1 at 31 years.Gestational smoke exposure is associated with increased incidence of adult-onset asthma and reduced pulmonary function in adult offspring.The results of this study are consistent with previous research revealing an association between gestational maternal smoke exposure and the subsequent development of asthma in offspring. This reveals susceptibility of the developing airway to maternal ETS exposure and the importance of smoking cessation during pregnancy. A limitation of this study is the inability to distinguish the role of prenatal maternal smoke exposure from maternal smoking in the home during the postnatal period.