Approximately 3% to 5% of children born in the United States manifest developmental defects. Between 2% and 3% of these birth defects can be classified as teratogen-induced malformations, which result from environmental or iatrogenic exposures during pregnancy. The final manifestations of drug-induced abnormal development can be death, malformation, growth retardation, or functional disorder. In considering the potential teratogenic effects of drugs, it is important to remember that susceptibility to a drug-induced malformation depends on: (1) the genotype of the conceptus, (2) the developmental stage at which exposure occurs, (3) the mechanisms of action of the drug, (4) the access of the drug to developing tissues, and (5) the dose of the drug. Nearly all teratogen-induced birth defects are preventable if the dose-response relationship for teratogen exposure and the teratogenic activity of the drug can be clearly defined. (J Allergy Clin Immunol 1999;103:S337-42.)