BACKGROUND Participants' understanding of clinical trials is important in informed consent. However, little is known about what information participants really want to know. OBJECTIVE To demonstrate the existence of a discrepancy between participants' understanding and their desire to know. METHODS The participants in clinical trials at Seoul National University Hospital were surveyed. The survey consisted of 11 statements based on the essential elements of informed consent. The participants gave two responses to each statement on a five-point Likert scale to rate their subjective understanding and desire to know, respectively. Information discrepancy was defined as the difference between these two ratings: if understanding exceeded desire to know for a particular item, it was defined as 'over-informed'; if desire to know exceeded understanding for a particular item, it was defined as 'under-informed'. RESULTS Participants reported good understanding of 'voluntariness', 'duration', 'study involves research' and poor understanding of 'confidentiality', 'compensation', 'benefits', 'procedures' and 'risks or discomforts'. For 'risks or discomforts', 'who to contact', 'voluntariness', 'duration' and 'procedures', participants reported high desire to know compared with 'confidentiality', 'purpose', 'study involves research' and 'benefits'. The elements 'study involves research', 'voluntariness', 'duration', 'purpose' and 'who to contact' were over-informed, while 'compensation', 'risks or discomforts', 'procedures', 'confidentiality' and 'benefits' were under-informed. Participants over 50 years of age, those without a college education and those whose participation was less voluntary were relatively less informed about the clinical trials. CONCLUSIONS An information discrepancy was observed between the participants' understanding and their desire to know. By putting more emphasis on under-informed elements, the quality of informed consent could be improved.