A number of studies have suggested that postnatal maternal cigarette smoking has a measurable effect on lung function in children. One longitudinal analysis of this problem in children 5-19 years of ages identified significant effects on the growth of FEV1 and forced expiratory flow between the 25th and 75th percent volume points (FEF25-75) of the volume-time curve. These investigators, however, pointed out that the effects which they observed could have resulted from the in utero consequences of maternal smoking during pregnancy and/or from exposure to factors such as severe respiratory illnesses which have been documented to occur with increased frequency early in infancy and were not measured by the study.