Background: This study assessed uptake of the Oncotype DX 21-gene assay over time and characterized which sociodemographic and clinical factors are associated with test uptake among women with lymph node−positive (LN+), hormone receptor−positive, HER2-negative breast cancer. Methods: Invasive breast cancer cases diagnosed in 2010 through 2013 were included from a SEER database linked to 21-gene assay results performed at Genomic Health’s Clinical Laboratory. Factors associated with 21-gene assay uptake were identified using a multivariable logistic regression model. Results: Uptake of the 21-gene assay increased over time and differed by race, socioeconomic status (SES), and age. In the multivariable model, when clinical and SES variables were controlled for, racial differences in test uptake were no longer observed. Private insurance status was associated with higher odds of 21-gene assay uptake (Medicaid vs private insurance: adjusted odds ratio, 0.86; P=.02), and high area-level SES was associated with an increased odds of uptake (quintile 5 vs 1: adjusted odds ratio, 1.6; P<.001). Demographic factors such as age and marital status influenced test uptake, and use varied greatly by geographic region. Uptake of the 21-gene assay increased over time and preceded the assay’s inclusion in the NCCN Guidelines for LN+ breast cancer. Differences in uptake by race, SES, and age have persisted over time. However, when clinical and SES variables were controlled for, racial differences in assay uptake were no longer observed. Socioeconomic variables, such as health insurance type and area-level SES, were associated with assay uptake. Conclusions: Future research should continue to document practice patterns related to the 21-gene assay. Given variation in testing associated with area-level SES, insurance coverage, and geographic region, interventions to understand and reduce differential uptake are needed to ensure equitable access to this genomic test.