Previous studies report mixed results about how much information study participants actually can read, understand and retain after completing the informed consent process; fewer studies have examined disparities in the retention and recall of information by patient factors, such as age, education, and race. Not retaining or being able to recall information from the informed consent process has potentially important ethical and legal implications and consequences for research quality and integrity, especially when found in populations that commonly are underserved or underrepresented in clinical trials. To determine how much basic knowledge participants finishing a five-year, multi-center, double-blinded randomized, placebo-controlled clinical trial had about the study, participants (n=1,789) were asked at their final follow-up visit three multiple-choice questions: (1) the study's purpose; (2) the name of the medication under investigation; (3) the main side effect of the medication. The associations between knowledge of these fundamental details and participant social and demographic factors were investigated. A majority of participants correctly recalled the study's purpose (64.7%) and medication (79.6%), but few correctly reported the main side effect (31.1%). In spite of relatively high recall for study purpose and medication, disparities by age, education and race exist. Increasing age was significantly associated with higher odds of incorrectly recalling both the study purpose and the name of the study medication. Likewise those with less than a high school education were more likely to incorrectly identify the study's purpose and the name of the study medication. Black and other non-white race or ethnic groups were more than two and a half times as likely to incorrectly identify the study's purpose. These findings suggest that even the most basic information may not be understood or retained by important subgroups of patients enrolled in clinical trials. Implementing effective strategies, such as additional time and effort for consent or repetition of study information, may be necessary in order to assure ethical and valid consent.