Case-control studies of antenatal drug exposure and birth defects often rely on maternal recall of drug use in pregnancy. Differential recall among mothers of cases and controls can lead to information bias ("maternal recall bias"), and the opportunity for such bias increases as ascertainment of drug exposure diminishes. The effect of questionnaire design on the ascertainment of drug use in pregnancy was examined in two studies. In a pilot interview study of 532 women in obstetric/gynecologic practices, information on drug use in the past year was obtained by means of three questions asked in sequence: The first question was open-ended, the second asked about drug use for selected indications, and the third asked about use of specifically named drugs. Among obstetric patients who reported use of any of five drugs, less than 50% did so in response to the open-ended question, and approximately 20-40% reported use only when the specific drug name was asked. In a case-control Birth Defects Study of 5,435 mothers of malformed children, information on drug use in pregnancy was obtained by asking questions in sequence about indications and specifically named drugs. Among the women who reported use of any of 11 drugs, 6-40% did so only when asked about the specific drug by name. These findings suggest that completeness of ascertainment of antenatal drug exposure varies according to how the mother is questioned and is directly related to the specificity of the questions asked.