This study systematically examined state-level laws protecting breastfeeding, including their current status and historical development, as well as identified gaps across US states and regions. The National Conference of State Legislatures summarised breastfeeding laws for 50 states and DC as of September 2010, which we updated through May 2011. We then searched LexisNexis and Westlaw to find the full text of laws, recording enactment dates and definitions. Laws were coded into five categories: (1) employers are encouraged or required to provide break time and private space for breastfeeding employees; (2) employers are prohibited from discriminating against breastfeeding employees; (3) breastfeeding is permitted in any public or private location; (4) breastfeeding is exempt from public indecency laws; and (5) breastfeeding women are exempt from jury duty. By May 2011, 1 state had enacted zero breastfeeding laws, 10 had one, 22 had two, 12 had three, 5 had four and 1 state had laws across all five categories. While 92% of states allowed mothers to breastfeed in any location and 57% exempted breastfeeding from indecency laws, 37% of states encouraged or required employers to provide break time and accommodations, 24% offered breastfeeding women exemption from jury duty and 16% prohibited employment discrimination. The Northeast had the highest proportion of states with breastfeeding laws and the Midwest had the lowest. Breastfeeding outside the home is protected to varying degrees depending on where women live; this suggests that many women are not covered by comprehensive laws that promote breastfeeding.