The School Breakfast Program is an important component of the nutritional safety net and has been linked to positive changes in meal patterns and nutritional outcomes. By offering a breakfast, which for low-income children is available either at no cost or reduced price, the program also has the potential to increase household food security. This study examined the relationship between availability of the School Breakfast Program and household food security among low-income third-grade students by using data from the Early Childhood Longitudinal Survey-Kindergarten Cohort. The primary sample included 3010 students. Availability of school breakfast was assessed by surveys of school administrators. Food security was assessed by parents' reports by using the standard 18-item food security scale and considering 2 different food security thresholds. A probit model was estimated to measure the relationship between school breakfast availability and household food security while controlling for a range of other characteristics. Access to school breakfast reduced the risk of marginal food insecurity but not the risk of food insecurity at the standard threshold. That is, the program appeared beneficial in offsetting food-related concerns among at-risk families, although not necessarily in alleviating food insecurity once hardships had crossed the food insecurity threshold. Increasing the availability of school breakfast may be an effective strategy to maintain food security among low-income households with elementary school children.