Purpose: Little is known about why women with breast cancer who have surgery do not receive proven effective postsurgical adjuvant treatments. Methods: We surveyed 258 women who recently underwent surgical treatment at six New York City hospitals for early-stage breast cancer about their care, knowledge, and beliefs about breast cancer and its treatment. As per national guidelines, all women should have received adjuvant treatment. Adjuvant treatment data were obtained from inpatient and outpatient charts. Factor analysis was used to create scales scored to 100 of treatment beliefs and knowledge, medical mistrust, and physician communication about treatment. Bivariate and multivariate analyses assessed differences between treated and untreated women. Results: Compared with treated women, untreated women were less likely to know that adjuvant therapies increase survival (on a 100-point scale; 66 v 75; P < .0001), had greater mistrust (64 v 53; P = .001), and had less self-efficacy (92 v 97; P < .05); physician communication about treatment did not affect patient knowledge of treatment benefits (r = 0.8; P = .21). Multivariate analysis found that untreated women were more likely to be 70 years or older (adjusted relative risk [aRR], 1.11; 95% CI, 1.00 to 1.13), to have comorbidities (aRR, 1.10; 95% CI, 1.04 to 1.12), and to express mistrust in the medical delivery system (aRR, 1.003; 95% CI, 1.00 to 1.007), even though they were more likely to believe adjuvant treatments were beneficial (aRR, 0.99; 95% CI, 0.98 to 0.99; model c, 0.84; P ≤ .0001). Conclusion: Patient knowledge and beliefs about treatment and medical mistrust are mutable factors associated with underuse of effective adjuvant therapies. Physicians may improve cancer care by ensuring that discussions about adjuvant therapy include a clear presentation of the benefits, not just the risks of treatment, and by addressing patient trust in and concerns about the medical system.