PURPOSE: For some women, adjuvant chemotherapy for nonmetastatic breast cancer decreases recurrences and increases survival; however, patient-physician decisions regarding chemotherapy receipt can be influenced by medical and nonmedical factors. PATIENTS AND METHODS: We used a prospective cohort design and multivariate modeling to investigate factors related to noninitiation of chemotherapy among women with newly diagnosed breast cancer recruited from three US sites. We interviewed patients at baseline and during treatment on sociodemographic, tumor, and treatment decision-making factors. Patients were categorized according to National Comprehensive Cancer Network guidelines as those for whom chemotherapy was definitely indicated, clinically discretionary, or discretionary based on age greater than 70 years. RESULTS: Of 1,145 patients recruited, chemotherapy was clinically indicated for 392 patients, clinically discretionary for 459 patients, discretionary because of age for 169 patients, and not indicated for 93 patients; data were insufficient for 32 patients. Chemotherapy rates were 90% for those in whom chemotherapy was clinically indicated, 36% for those in whom it was discretionary because of clinical factors, and 19% for those in whom it was discretionary based on age greater than 70 years. Nonreceipt of chemotherapy was associated with older age, more negative beliefs about treatment efficacy, less positive beliefs about chemotherapy, and more concern about adverse effects. In the two discretionary groups, clinical predictors of worse outcome (greater tumor size, positive nodes, worse grade, and estrogen receptor- and progesterone receptor-negative status) were associated with increased chemotherapy initiation. CONCLUSION: Utilization of adjuvant chemotherapy was most common among patients who, based on clinical criteria, would most likely benefit from it, patients with more positive than negative beliefs regarding treatment efficacy, and patients with few concerns about adverse effects.