Abstract Background Women with newly diagnosed breast cancer face multiple treatment options. Involving them in a shared decision-making (SDM) process is essential. The aim of this study was to evaluate whether a multilevel implementation programme enhanced the level of SDM behaviour of clinicians observed in consultations. Methods This before–after study was conducted in six Dutch hospitals. Patients with breast cancer who were facing a decision on surgery or neoadjuvant systemic treatment between April 2016 and September 2017 were included, and provided informed consent. Audio recordings of consultations made before and after implementation were analysed using the five-item Observing Patient Involvement in Decision-Making (OPTION-5) instrument to assess whether clinicians adopted new behaviour needed for applying SDM. Patients scored their perceived level of SDM, using the nine-item Shared Decision-Making Questionnaire (SDM-Q-9). Hospital, duration of the consultation(s), age, and number of consultations per patient that might influence OPTION-5 scores were investigated using linear regression analysis. Results Consultations of 139 patients were audiotaped, including 80 before and 59 after implementation. Mean (s.d.) OPTION-5 scores, expressed on a 0–100 scale, increased from 38.3 (15.0) at baseline to 53.2 (14.8) 1 year after implementation (mean difference (MD) 14.9, 95 per cent c.i. 9.9 to 19.9). SDM-Q-9 scores of 105 patients (75.5 per cent) (72 before and 33 after implementation) were high and showed no significant changes (91.3 versus 87.6; MD −3.7, −9.3 to 1.9). The implementation programme had an association with OPTION-5 scores (β = 14.2, P < 0.001), hospital (β = 2.2, P = 0.002), and consultation time (β = 0.2, P < 0.001). Conclusion A multilevel implementation programme supporting SDM in breast cancer care increased the adoption of SDM behaviour of clinicians in consultations.