Objectives Patients' negative treatment expectations can lead to nocebo‐related side effects and non‐initiation of treatment. This study aims to identify correlates of treatment expectations in patients with breast cancer before the start of endocrine therapy. Methods Expectations were assessed in a cross‐sectional sample of 166 patients with breast cancer after receiving treatment information. Side effect expectations (one item) and treatment necessity–concern balance (Beliefs about Medicines Questionnaire) were assessed. Correlates were analyzed using regression analyses. The structure of treatment expectations was investigated using a network analysis. Results About 25% of patients expressed negative expectations. Higher side effect expectations were associated with lower treatment efficacy expectations (ß = −0.20, p = 0.01), higher medication overuse beliefs (ß = 0.17, p = 0.01), and a negative treatment appraisal before study treatment information (ß = −0.17, p = 0.02). A negative necessity–concern balance was associated with lower treatment efficacy expectations (ß = 0.36, p < 0.001), lower adherence intention (ß = 0.21, p < 0.001), and no knowledge of tumor's receptor status (ß = 0.21, p < 0.001); furthermore, it was associated with higher medication harmfulness beliefs (ß = −0.16, p = 0.02), negative treatment pre‐appraisal (ß = 0.15, p = 0.01), higher somatosensory amplification (ß = −0.14, p = 0.02), and higher education (ß = −0.12, p = 0.02). The most important network node was the concern that endocrine therapy disrupts life. Conclusion Negative treatment expectations before treatment start are mainly associated with psychological variables. These results are relevant for patient education in clinical settings. To improve expectations, clinicians might emphasize treatment efficacy and discuss general and specific medication concerns. Improving treatment knowledge could also be beneficial. Copyright © 2016 John Wiley & Sons, Ltd.