It is necessary to carry out randomised clinical cancer trials (RCTs) in order to evaluate new, potentially useful treatments for future cancer patients. Participation in clinical trials plays an important role in determining whether a new treatment is the best therapy or not. Therefore, it is important to understand on what basis patients decide to participate in clinical trials and to investigate the implications of this understanding for optimising the information process related to study participation. The aims of this study were to (1) describe motives associated with participation in RCTs, (2) assess if patients comprehend the information related to trial enrolment, and (3) describe patient experiences of trial participation. Questionnaires were sent to 96 cancer patients participating in one of nine ongoing clinical phase 3 trials at the Department of Oncology, Uppsala University Hospital in Sweden. Eighty-eight patients completed the questionnaire (response rate 92%); 95% of these were patients in adjuvant therapy and 5% participated in clinical trials on palliative care. Two main reasons for participation were identified: personal hope for a cure and altruism. Patients show adequate understanding of the information provided to them in the consent process and participation entails high patient satisfaction.