Objective: This study's aim was to determine how patients' and providers' characteristics affect hospital providers' decisions to screen pregnant and postpartum women for illicit substances. Methods: A retrospective design was used. Participants included all low-income women (N=1,100) who delivered at an urban teaching hospital over a 12-month period and the providers (N=40) who provided prenatal and delivery care for these women. The medical records of the women were abstracted to obtain demographic, medical, social, and substance use information. Providers were interviewed to obtain data on their attitudes. The outcome was a dichotomous indication of whether patients were screened for illicit substances. Results: Multivariate logistic regression analyses found that women who were single (OR=7.1), Black (OR=1.9), received prenatal care at the prenatal clinic (OR=5.6), saw fewer providers (1.1), or had a placental abruption (OR=15.8), preterm labor (OR=3.0), inadequate prenatal care (OR=4.9), a history of involvement with Child Protective Services (OR=3.9), a high social/Mental Health Risk Factor Score (OR=1.4), a past or present history of illicit drug use (OR=6.7), or a present history of tobacco use (OR=1.7) were more likely to be screened than women without these characteristics. Women whose providers scored medium (OR=2.5) or high (OR=2.5) on the Professionalism Scale were more likely to be screened than women whose providers scored low on this scale. Conclusions: Providers' decisions to screen pregnant women for illicit substance use are influenced by both patients' characteristics and providers' personal attitudes. Hospital protocols might help reduce the potentially biased impact of attitudes on screening decisions.