Objective: A patient decision aid (PtDA) is often developed and evaluated to support shared decision making (SDM) and a patient-centred approach. In this study, a PtDA template was developed to support two different preference sensitive decisions: adjuvant therapy for breast cancer and diagnostic workup for lung cancer. The aim of the study was to explore whether a PtDA improved SDM and supported a patient-centred approach from an observational point of view. Methods: Real-life observations were conducted using the validated observational instrument OPTION 12. Three nurses conducted observations of consultations in two different clinical settings. The study consisted of a baseline cohort (phase 1) and an intervention cohort (phase 2). In phase 1, standard consultations were observed. Subsequently, the PtDA was introduced, and in phase 2, consultations with the clinician using the PtDA were observed. Results: Twenty-nine observations were conducted in phase 1 and 24 in phase 2. Using a PtDA increased the overall OPTION score significantly (P <.0001), both in decisions on adjuvant treatment for breast cancer and on diagnostic workup of lung cancer. Items in the OPTION instrument regarding systematic conversation to obtain SDM had a higher increase of scores compared to items regarding a patient-centred approach. Conclusion: Real-life observations and the use of a validated observational tool provided comprehensive knowledge as to how a PtDA affects SDM in consultations. Applying a PtDA resulted in a significant overall increase of SDM behaviour in decisions on adjuvant treatment after breast cancer surgery and diagnostic workup in case of a small suspicion of lung cancer. In conclusion, the PtDA supports SDM in consultations independently of type of decision and department.