The aim of this study was to test an instrument which might be useful for doctors in explaining the randomisation procedure to an individual patient. The sample comprised 323 patients with cancer attending for out-patient appointments and/or chemotherapy treatment in two major cancer centres in the U.K. 315 patients completed a self-report questionnaire--The Attitudes to Randomised Trials Questionnaire (ARTQ). The results show that the majority of subjects 287 (91.1%) believe that patients should be asked to take part in medical research, but only 242 (76.8%) would be prepared to take part in a study comparing two treatments. If treatment was randomised, only 141 (44.8%) would agree to participate. When given further information about the randomisation procedure, 119 (68.4%) of the 174 (55.2%) who initially said 'no' to randomisation or who were unsure, would change their minds and take part in a trial. The ARTQ discriminated between three categories of patient with the following prevailing attitudes: (a) those who seem comfortable with the concept of randomisation; (b) those with some concerns, who with fuller explanation are prepared to consider randomisation; and (c) those firmly against randomisation and participation in trials whatever information is provided. Prior knowledge of patients' attitudes might assist communication about trials and encourage more doctors to approach eligible patients.