Adherence to antiretroviral therapy (ART) in pregnancy is crucial to optimize its efficacy and minimize mother-to-child transmission. Our objective was to examine adherence patterns to ART and health behaviors during and after pregnancy among HIV-positive women enrolled in A5084, a prospective, observational, multisite study. Between 2002-2005, HIV-infected women between 20 and 34 weeks'gestation completed at least 1 self-reported adherence questionnaire antepartum (AP), and were followed through 12 weeks' postpartum (PP). Questionnaires also addressed tobacco, alcohol, and illicit drugs use. Adherence was defined as reporting not having missed any doses for more than 3 months. Exact McNemar's tests were used for paired binary data and exact logistic regression was used for predictors of nonadherence. We report on 149 women (55% black, 26% Hispanic, 32% less than 25 years, 9% with AIDS, 100% on ART). PP, 31 (21%) women stopped ART and 18 (12%) withdrew from the study. AP, 57% reported adherence to ART and PP, 45% (p = 0.03, n = 87). AP, 11% reported ongoing alcohol use and 23% tobacco use compared to 37% and 30% PP (p < 0.0001, n = 103; p = 0.07, n = 99, respectively). Although 39% ever used marijuana (n = 116) and 25% used illicit drugs (n = 107), few participants reported use during the study. In multivariate analyses, those who had ever used illicit drugs had 5.95 times higher odds (p = 0.002) and those who missed prenatal vitamins had 4.84 times higher odds (p = 0.001) of ART nonadherence. Women reporting a history of illicit drug use and/or having missed prenatal vitamins should be targeted for programs to enhance adherence to ART during pregnancy.