The effects of pre- or postnatal passive smoking on the adult incidence of asthma have not been reported previously. Between 1985 and 1996/1997, we conducted an 11-year community cohort study on the incidence of asthma and respiratory symptoms in Western Norway. The cohort included 3,786 subjects aged 15 to 70 years, of which 2,819 were responders at both baseline and follow-up. The incidence of asthma and five respiratory symptoms by self-reported exposure to maternal smoking in utero and in childhood, as well as smoking by other household members in childhood, was examined. After adjustment for sex, age, education, hay fever, personal smoking, and occupational exposure, maternal smoking was associated with asthma, phlegm cough, chronic cough, dyspnea grade 2, attacks of dyspnea, and wheezing, with odds ratios (95% confidence intervals [CI]) of 3.0 (1.6, 5.6), 1.7 (1.1, 2.6), 1.9 (1.2, 3.0), 1.9 (1.2, 3.0), 2.0 (1.3, 3.0), and 1.4 (0.9, 2.2), respectively. The adjusted attributable fractions (95% CI) of the adult incidence of asthma were 17.3% (5.2, 27.9) caused by maternal smoking and 9.3% (95% CI, -23.2, 33.2) caused by smoking by other household members. Exposure to pre- and postnatal smoking carries a substantial risk for developing adult asthma and respiratory symptoms.