The children's smoking behavior was divided into five categories: never having smoked, having smoked formerly, or currently smoking 1/2 to 4, 5 to 14, or 15 or more cigarettes per day. Maternal smoking status, found previously to predict the level of lung function in this cohort,11 was modeled with the use of an indicator for former smoking and current smoking measured as the number of cigarettes per day. When relations between smoking and lung function were linear, regression models were considered that included the number of cigarettes smoked per day as a continuous variable and former smoking as a categorical variable. The mean education of the parents was categorized as less than, equal to, or greater than 12 years.