The relationship between parental smoking and both subsequent development of asthma and subsequent lung function (before age 12) was studied in more than 700 children enrolled before age 5. Children of mothers with 12 or fewer years of education and who smoked 10 or more cigarettes per day were 2.5 times more likely (95% confidence interval 1.42 to 4.59; P = .0018) to develop asthma and had 15.7% lower maximal midexpiratory flow (P less than .001) than children of mothers with the same education level who did not smoke or smoked fewer than 10 cigarettes per day. These relationships were independent of self-reported respiratory symptoms in parents. There was no association between maternal smoking and subsequent incidence of asthma or maximal midexpiratory flow among children of mothers with more than 12 years of education. It is concluded that children of lower socioeconomic status may be at considerable risk of developing asthma if their mothers smoke 10 or more cigarettes per day. It is speculated that recently reported increases in prevalence of childhood asthma may be in part related to the increased prevalence of smoking among less educated women.