OBJECTIVES: The 21-gene recurrence score assay (RS) (Oncoytpe Dx, Genomic Health, Redwood City, CA) is a genomic test guiding adjuvant chemotherapy use in patients with early-stage breast cancer.  We assessed the association between RS receipt and subsequent chemotherapy use and medical expenditures among newly diagnosed early-stage breast cancer patients with commercial insurance.  We also explored whether the effects of RS receipt varied by patient age. METHODS: The 21-gene recurrence score assay (RS) is a genomic test guiding adjuvant chemotherapy use in patients with early-stage breast cancer.  We assessed the association between RS receipt and subsequent chemotherapy use and medical expenditures among newly diagnosed early-stage breast cancer patients with commercial insurance.  We also explored whether the effects of RS receipt varied by patient age. RESULTS: RS receipt ranged from 40% among women <45 years to 47% among women 60-64 years (p=0.54).  Chemotherapy use declined from 68% among women <45 years to 37% among women 60-64 years (p<0.001).  RS receipt was associated with lower adjusted chemotherapy use among women <55 years (age <45: 61% vs. 73%, p=0.14; age 45-49: 33% vs. 61%, p<0.001; age 50-54: 36% vs. 55%, p=0.010), but not among women ≥55 years (age 55-59: 48% vs. 49%, p=0.83; age 60-64: 41% vs. 40%, p=0.86).  RS receipt also had a larger impact on adjusted medical spending among women <55 years (age <45: $102,000 vs. $130,800, p=0.046; age 45-49: $75,700 vs. $104,600, p=0.001; age 50-54: $78,100 vs. $91,400, p=0.059) than women ≥55 years (age 55-59: $87,100 vs. $91,600, p=0.61; age 60-64: $78,300 vs. $83,100, p=0.48). CONCLUSIONS: Among adult women with commercial insurance, RS receipt was associated with less use of adjuvant chemotherapy and lower medical spending among women <55 years old but not among women 55-64 years old.