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To improve the patient education process in clinical research, three information materials describing general aspects of design and conduct of randomized clinical trials were developed. The materials varied in length, reading ability level, and reader appeal. Their influence on knowledge about and attitude toward randomized clinical trials was assessed in a randomized, parallel group, evaluator-blinded trial among 415 outpatients. The patients were randomized to the following groups: control (no intervention), leaflet, brochure, or booklet. Knowledge was assessed by a 17-item multiple-choice questionnaire and attitude was assessed by a 32-item Likert questionnaire at entry and 2 weeks after the intervention. The interventions and the questionnaires were pilot tested and power calculations were performed. At entry, the mean knowledge score was 7.9 points. At follow-up, the knowledge scores increased by 0.5 for the control, 1.0 for the leaflet, 1.6 for the brochure, and 1.4 for the booklet. The brochure and the booklet improved the knowledge score significantly compared with the control. The general attitude was positive at entry (mean 71.5 points). Only the booklet significantly increased the total attitude score (4.8 points) and the randomized clinical trials attitude subscale score (1.8 points). In conclusion, written information significantly improved outpatients' knowledge about and attitude toward randomized clinical trials. Detailed rather than brief information was more effective. Control Clin Trials 2000;21:223-240