Many patients with haematological malignancies experience barriers in clinical communication. Reaching effective communication is of great importance as it has been linked to a range of improved patient outcomes such as satisfaction, compliance to treatment, perceived quality of life and physical and mental health. To get a better understanding how communication in haematological consultations can be improved, the current study focussed on patients' preferences and perceived performances regarding the communicative behaviour of their health care professional. Secondly, the mediation of an online communication tool for patients was analysed. Within a controlled pre- post-test design, 78 datasets of clinical consultations could be analysed. Patients considered both affective and instrumental communication aspects important. The affective communication behaviour of the health care professional met the patients' pre-visit preferences well. In the information exchange, more variability and discrepancies were found. Overall, the online intervention did not seem to influence the patients' perceived communication performance of their health care professional much. To further improve the communication during clinical consultations, health care professionals should inquire about patients' expectations, especially during the exchange of information and advices. At the same time, patients should be supported to express their preferences at the start of the consultation. The study was registered in the Netherlands Trial Register, number 3779.