BACKGROUND AND PURPOSE: Informed consent from trial participants is mandatory. In a randomized clinical trial, we investigated (1) differences in knowledge and understanding of trial information between patients who participated and those who refrained, (2) differences in perceptions of information, and (3) differences in satisfaction with the information.

PATIENTS: After the decision about participation in the randomized study, 'Surgery versus radiotherapy for locally advanced prostate cancer' (SPCG-15), patients were sent questionnaires ('Quality of Informed Consent', EORTC QLQ-INFO25). Patients were categorized in 'Non-participants' or 'Participants'.

RESULTS AND INTERPRETATION: A total of 80 patients (80%) responded, 68% of non-participants and 95% of participants. Between-group differences in knowledge were found for duration of the trial, insurances in the trial, and if the trial intervention had been proven to be superior. Patients had high levels of knowledge (> 80%) regarding the trial aim, that participation implied research, the right to decline, that future patients benefit from research and, of the randomization procedure. Less than 50% responded correctly concerning risks associated with the trial, the unproven nature of the trial and issues about insurances. Non-participants scored lower concerning duration of trial participation, confidentiality of medical records, treatments and procedures in the trial, and experimental nature of treatments. There were no differences regarding satisfaction with information. Non-participants and participants did not differ in satisfaction, or in knowledge and understanding of most aspects of the information. Knowledge levels were low in some areas, and thus, it seems to be room for improvement to fulfill the requirements of informed consent.