e23048 Background: Exercise is associated with improved symptoms and breast cancer (BC) survival. Little is known about personal and social factors that determine exercise participation in diverse BCS. The study seeks to characterize exercise trends and distinguish factors associated with initiating exercise among pre-diagnosis employed, non-exercising, ethnically and socioeconomically diverse BCS. Methods: BCS were recruited from academic and community medical centers. Women with stage I-III BC were surveyed during and 4 months post-completion of active treatment (surgery +/- chemo +/- radiation). The primary outcome was exercise initiation 4 months post treatment. Variables were self-reported except for clinical cancer variables, which were abstracted from the medical record. Results: Our sample (n = 494) included 22% black, 20% Chinese, 8% Korean, 27% Latina, and 21% non-Latina white women (3% other). 56% were born outside of the US; 30% reported an income < 200% of the federal poverty level (FPL). 72% of BCS exercised the year before diagnosis; 28% did not. Significant correlates (p < 0.05) of NOT exercising pre-diagnosis included lack of post-secondary education, Chinese or Korean ethnicity, lack of alcohol consumption, and lower acculturation level, resilience, and income. Among 138 non-exercisers, 63% reported exercising at follow-up. Significant correlates of exercise initiation are listed in table. In a multivariable model that included age, acculturation, job loss, insurance status, chemo-, and radiotherapy (RT), receipt of RT was significantly associated with exercise initiation (OR 3.1, 95% CI 1.4-7.1). Conclusions: A BC diagnosis may be an impetus to initiate exercise among previously sedentary, employed women. Patients who undergo radiotherapy appear to be more likely to start exercising. Additional research is needed to understand why some patients initiate exercise while others do not. [Table: see text]