Asthmatic mothers have been reported to deliver infants of lower mean birth weight than nonasthmatic mothers. This study examined the relationship between intrauterine growth and serial gestational spirometry in 352 pregnant asthmatic women who were prospectively treated and observed during pregnancy. A small (r = 0.11) but significant (p less than 0.04) direct correlation was demonstrated between infant birth weight and individual mean percent predicted FEV1 during pregnancy. In addition, lower maternal mean FEV1 during pregnancy was associated with increased incidences of birth weight in the lower quartile of the infant population (p = 0.002) and ponderal indices less than 2.2 (suggestive of asymmetric intrauterine growth retardation) (p less than 0.05), but not with increased incidences of preterm (less than 38 weeks) or low birth weight (less than 2,500 g) infants. Although lower mean birth weight occurred in infants of smoking compared with nonsmoking asthmatic mothers (p less than 0.02), the relationships of lower FEV1 to birth weight in the lower quartile of the population (odds ratio 3.0, p = 0.002) and ponderal indices less than 2.2 (odds ratio 2.8, p less than 0.05) were shown by multivariate analysis to be above and beyond the influence of smoking and also independent of the effects of age, parity, acute asthmatic episodes, and asthma medications. These data support the hypothesis that lower maternal gestational FEV1 during pregnancy is related to intrauterine growth retardation and suggest that the goals of gestational asthma therapy should include optimization of pulmonary function in addition to achievement of symptomatic control.