Lack of understanding or knowledge of research among patients participating in randomized clinical trials (RCTs) is common. However, there is a lack of validated measures to assess the extent of the problem. The aim of this study was to develop and pilot a questionnaire to assess knowledge and understanding of RCTs in the cancer setting. A 13-item four-part questionnaire was developed and piloted with three groups: patients with cancer who had no experience of clinical trials (n=26); patients with cancer who had previously participated in an RCT (n=26) and research nurses (n=26). The groups were compared using non-parametric anova techniques which showed a highly statistically significant difference between the nurses and both patient groups (P<0.001). The difference between the patient groups was not significant (P=0.231). Cronbach's alpha was computed at 0.77. After studying correlation of individual questions and verbatim comments from respondents, one question was removed from the questionnaire, but this did not have a major effect on the properties of the tool. Minor changes were made to six other questions and the revised tool will now be used and further tested in a large randomized controlled trial to improve informed consent in trial participation.