Pregnancy can affect the course of asthma, and asthma can affect pregnancy outcomes. Asthma course may improve, worsen, or remain unchanged during pregnancy. The mechanisms involved have not been defined, and the course in an individual woman is largely unpredictable. Therefore pregnant women with asthma must be followed up particularly closely so that any change in course can be matched by an appropriate change in therapy. The data also suggest that asthma is associated with increased frequencies of adverse pregnancy outcomes. The mechanisms of this are not totally clear either, but some data support the hypothesis that improved pregnancy outcomes are associated with well-controlled disease. The risk of using medications to control asthma during pregnancy appears to be much less than the risk of adverse outcomes related to severe uncontrolled asthma. (J Allergy Clin Immunol 1999;103:S330-6.)