The aim of this study was to determine the communicative needs of the patients in the context of being invited to participate in a clinical trial. A questionnaire was sent to 299 patients with breast cancer randomised in a trial of adjuvant therapy. It was returned by 261 (87%) of them. Ninety-one per cent (231/255) of the patients regarded the information provided as easy or quite easy to understand. However, the method of treatment allocation was unclear to most patients: 51% (128/251) thought that the doctor had chosen the treatment while only 23% (57/251) knew that they had been randomised. Younger and better educated patients had a better understanding. For 55% (125/226) of the patients written information had been helpful in decision making. This correlated highly with the education of the patient. Sixty-eight per cent (174/255) of the patients thought that they had enough time for decision-making. Less educated patients and older patients had needed more time. Eighty-seven per cent (218/251) were happy with their decision to participate. While most patients are satisfied with the information received, there is a poor understanding of how treatment is allocated. Information should be modified for older and less-educated patients. The needs of the patients when offered participation in a clinical trial are clear information, enough time to consider the options and psychological support.