Background: A number of practice guidelines incorporate the use of gene expression profiling (GEP) tests for early-stage, hormone receptor–positive, HER2-negative breast tumors. Few studies describe factors associated with GEP testing in US oncology practice. We assessed the relationship between clinical, demographic, and group-level socioeconomic variables and test use in women younger than 65 years. Patients and Methods: Data from 5 state cancer registries were linked with insurance claims data and GEP test results. We assessed rates of testing and variables associated with test use in an incident cohort of 9,444 commercially insured women younger than 65 years, newly diagnosed with stage I or II hormone receptor–positive breast cancer from 2006 through 2012. Results: Rates of testing for women with N0 disease increased from 20.4% in 2006 to 35.2% in 2011. Variables associated with higher rates of testing, beyond clinical factors such as nodal status (P<.001), included being diagnosed from 2008 through 2012 versus 2006 through 2007 (adjusted odds ratio [OR], 1.67; 95% CI, 1.47–1.90), having preexisting comorbidities (adjusted OR, 1.35; 95% CI, 1.14–1.59), and higher out-of-pocket pharmacy costs (adjusted OR, 1.66; 95% CI, 1.40–1.97). Women younger than 50 years were more likely to be tested if they had stage I versus stage II disease (P<.0001). Conclusions: In an insured population of women younger than 65 years, GEP testing increased after its inclusion in clinical practice guidelines and mounting evidence. Additional research is needed to better understand oncologists' decision not to order GEP testing for their patients who are otherwise eligible.