OBJECTIVES: Although breast cancer occurs primarily among women over age 50 years, each year more than 10,000 cases are diagnosed among women aged 18-44 years.  To date, no study has specifically quantified the economic burden of breast cancer in this age group in the United States.  This study is the first to estimate work and home productivity losses due to breast cancer among younger women aged 18-44 years. METHODS: We used a two-part regression model and data from the 2000–2010 National Health Interview Survey to estimate the number of work and home productivity days missed due to breast cancer, adjusted for socioeconomic characteristics and comorbidities. We compared our estimates for younger women with those of older women aged 45-64 years. RESULTS: Per capita, younger women with breast cancer had annual losses of $2,293 (95% confidence interval [CI]: $1,069-$3,518) from missed work and $288 (95% CI: $105-$471) from missed home productivity. Younger women too sick to work had annual breast cancer-attributable work loss costs of $365 per person (95% CI: $208-$522). In total, breast cancer-attributable morbidity costs for younger women were $274 million (95% CI: $128-$420 million) for work loss among the employed, $46 million (95% CI: $17-$75 million) for home productivity, and $10 million (95% CI: $6-$15 million) for work loss among women too sick to work.  Older women with breast cancer had lower per capita work loss costs but higher total morbidity costs. CONCLUSIONS: Younger women with breast cancer face significant work and home productivity losses. These results underscore the importance of continued efforts by the public health community to support the unique needs of younger breast cancer survivors and promote prevention efforts throughout the lifespan.