Some population-based studies of the risks of gestational medication use are limited by differential recall or enrollment of mothers of children with abnormalities. Prospective acquisition of exposure information, before knowledge of pregnancy outcome, is the most unbiased manner in which to obtain denominator-based outcome information. The Organization of Teratology Information Services (OTIS) Registry Study will prospectively evaluate pregnancy outcomes in relation to exposures to asthma medications. Patients will be enrolled by participating Teratology Information Services. Information regarding medication exposure, asthma severity, and other important medical events will be assessed during pregnancy by means of standardized interviews, supplemented by medical records. Outcome information will be obtained from copies of the pediatric records. Data analysis will evaluate relative risks of adverse outcomes in exposed compared with unexposed asthmatic and nonasthmatic subjects.