Objective The objective of this study is to investigate young breast cancer patients' preferred and actual involvement in decision‐making about surgery, chemotherapy, and adjuvant endocrine therapy (AET). Methods A total of 442 women aged 18–40 years at the time of the diagnosis participated in the region‐wide ELIPPSE40 cohort study (southeastern France). Logistic regression analyses were performed on various factors possibly affecting patients' preferred and perceived involvement in the decisions about their cancer treatment. Results The women's mean age was 36.8 years at enrolment. Preference for a fully passive role in decision‐making was stated by 20.7% of them. It was favored by regular breast surveillance (p = 0.04) and positive experience of being informed about cancer diagnosis (p = 0.02). Patients' preferences were independently associated with their reported involvement in decision‐making about surgery (p = 0.01). A fully passive role in decision‐making about chemotherapy and AET was more likely to be reported by patients who perceived their involvement in decision‐making about surgery as having been fully passive (adjusted odds ratio = 4.8, CI95% [2.7–8.7], and adjusted odds ratio = 9.8, CI95% [3.3–29.2], respectively). This study shows a significant relationship between the use of antidepressants and involvement in decision‐making about surgery, and confirms the relationship between impaired quality of life (in the psychological domain) and a fully passive role in decisions about cancer treatment. Conclusions Patients' involvement in decision‐making about chemotherapy and AET was strongly influenced by their experience of decision‐making about surgery, regardless of their tumor stage and history of breast or ovarian cancer. When decisions are being made about surgery, special attention should be paid to facilitating breast cancer patients' involvement in the decision‐making. Copyright © 2013 John Wiley & Sons, Ltd.