BACKGROUND Concern has been expressed about the process of consent to clinical trials, particularly in phase I "first-in-man" trials. Trial participant information sheets are often lengthy and technical. Content-based readability testing of sheets, which is often required to obtain research ethics approval for trials in the USA, is limited and cannot indicate how information will perform. METHODS An independent-groups design was used to study the user-testing performance of the participant information sheet from the phase I TGN1412 trial. Members of the public were asked to read it, then find and demonstrate understanding of 21 key aspects of the trial. The participant information sheet was then rewritten, redesigned and tested on 20 members of the public, using the same 21-item questionnaire. RESULTS On the original TGN1412 participant information sheet, participants could not find answers and some of the found information was not understood. Six of 21 questions, including those relating to placebo, follow-up visits and the emergency phone number, were found by eight or fewer of 10 participants. The revised information sheet performed better, with the answers to 17 of 21 questions found and understood by all 20 participants. CONCLUSIONS Tests showed that the TGN1412 participant information sheet may not inform participants adequately for consent. Revising its content and design led to significant improvements. Writers of materials for trial participants should take account of good practice in information design. Performance-based user testing may be a useful method to indicate strengths and weaknesses in trial materials.