Factors that affect airway growth-as early in development as in utero-seem to cause physiologic effects that can be persistent. Reduced airway function early in life does not necessarily result in persistent symptoms, but it does increase the risk of reduced lung function and the development of persistent airflow limitation in adult life, both in men and women. Normal lung growth varies with age and sex and is affected by a number of risk factors, which we have described. The importance of the various risk factors may differ depending at what point during lung growth they come into play and whether they occur in men or in women.