OBJECTIVE: To compare patients' perceptions and preferences of two different versions of informed consent documents. METHODS: Patients eligible for a trial of palliative chemotherapy for lung cancer (N = 22) were randomly assigned to receive either an original consent document or a shortened version written for the present study. Semi-structured interviews were conducted after the patients had read the consent documents. The interviews were transcribed verbatim and analysed using qualitative content analysis. RESULTS: Few differences between the two groups were found with respect to patients' assessment of the amount of content and the most important information in the documents. Information about disease and treatment seemed to be of most interest for the patients, while information about research aspects of the study such as financing, confidentiality and publishing (formalities) was judged to be of lesser relevance. Two patients who read the original document indicated that they treated the formalities as secondary. CONCLUSION: Patients seemed to pay little attention to the research aspects, and thus risked to misunderstand the main point of the consent document. PRACTICE IMPLICATIONS: The structure of consent documents should clarify for the readers that they are asked to take part in research, and that participation is voluntary.