1. In order to assess attitudes of patients to participation in therapeutic trials 197 patients underwent structured interviews conducted by a single observer. Of these patients 66 were previous or current participants in clinical trials (group A), 12 had declined participation in a trial (group B), and 119 had never been invited to participate in research (group C). 2. In group A, 62% stated their motivation for participation was to help others and 39% to improve their own treatment, but in 38% participation was to comply with the doctor's request. Two-thirds of group C patients would or might participate in a future hypothetical trial; of these more than half (57%) would do so to help others, and 42% to improve their own treatment. 3. Of the 12 patients who had declined entry into a study (group B) three did not want to alter their current therapy, three had insufficient time to participate in the particular trial, and in three cases relatives objected to their participation. Group C patients who stated they would not participate in trials gave being too ill (22%), not wanting to change treatment (22%), and fear of side-effects (17%) as the commonest reasons for declining. 4. In group A, 83% felt they had adequate time to consider their participation. Nearly two-thirds of patients (60%) would have liked written information to retain for reference, whereas only 38% were provided with information in this form. Over half of these patients (54%) disliked no aspect of the study in which they participated. Venepuncture and other uncomfortable procedures were least popular.(ABSTRACT TRUNCATED AT 250 WORDS)