OBJECTIVE To examine the effect of nurse staffing in varying work environments on missed breastfeeding teaching and support in inpatient maternity units in the United States. BACKGROUND Breast milk is the optimal food for newborns. Teaching and supporting women in breastfeeding are primarily a nurse's responsibility. Better maternity nurse staffing (fewer patients per nurse) is associated with less missed breastfeeding teaching and support and increased rates of breastfeeding. We examined the extent to which the nursing work environment, staffing, and nurse education were associated with missed breastfeeding care and how the work environment and staffing interacted to impact missed breastfeeding care. METHODS In this cross-sectional study using the 2015 National Database of Nursing Quality Indicator survey, maternity nurses in hospitals in 48 states and the District of Columbia responded about their workplace and breastfeeding care. Clustered logistic regression models with interactions were used to estimate the effects of the nursing work environment and staffing on missed breastfeeding care. RESULTS There were 19 486 registered nurses in 444 hospitals. Nearly 3 in 10 (28.2%) nurses reported missing breastfeeding care. In adjusted models, an additional patient per nurse was associated with a 39% increased odds of missed breastfeeding care. Furthermore, 1 standard deviation decrease in the work environment was associated with a 65% increased odds of missed breastfeeding care. In an interaction model, staffing only had a significant impact on missed breastfeeding care in poor work environments. CONCLUSIONS We found that the work environment is more fundamental than staffing for ensuring that not only breastfeeding care is not missed but also breastfeeding care is sensitive to nurse staffing. Improvements to the work environment support the provision of breastfeeding care. CONCLUSIONS Both nurse staffing and the work environment are important for improving breastfeeding rates, but the work environment is foundational.