Despite the documented health and emotional benefits of breast-feeding to women and children, breast-feeding rates are low among subgroups of women. In this study, we examine factors associated with breast-feeding initiation in low-income women, including Theory of Planned Behavior measures of attitude, support, and perceived control, as well as sociodemographic characteristics. A mail survey, with telephone follow-up, of 733 postpartum Medicaid beneficiaries in Mississippi was conducted in 2000. The breast-feeding initiation rate in this population was 38%. Women who were older, white, non-Hispanic, college-educated, married, not certified for the Supplemental Nutrition Program for Women, Infants, and Children, and not working full-time were more likely to breast-feed than formula-feed at hospital discharge. Attitudes regarding benefits and barriers to breast-feeding, as well as health care system and social support, were associated with breast-feeding initiation at the multivariate level. Adding the health care system support variables to the regression model, and specifically support from lactation specialists and hospital nurses, explained the association between breast-feeding initiation and women's perceived control over the time and social constraints barriers to breast-feeding. The findings support the need for health care system interventions, family interventions, and public health education campaigns to promote breast-feeding in low-income women.