Purpose: To identify barriers to, and motivations for, recruitment and retention in osteoporosis related clinical trials among postmenopausal women. Methods: We explored the self reported reasons for and against participation in clinical trials among women who expressed an interest in participating in the Nitrates and Bone Turnover (NABT) study: an ongoing randomized controlled trial based at an urban tertiary care centre (Women's College Hospital, University of Toronto). The study was designed to compare the effects of different doses and formulations of nitrates on markers of bone turnover among postmenopausal women not diagnosed and/or receiving treatment for osteoporosis. We administered a standardized interviewer questionnaire to 53 women to determine their reasons for participation in the NABT trial. To determine reasons for non-participation, we administered a questionnaire to 9 women and reviewed data collected at the time of initial assessment in 56 women who were not interested in participating in the trial. We conducted qualitative analyses using thematic coding of these responses. Results: The most common reasons for participation were: altruism (26.4%) and potential personal benefits (22.6%). The two most common reasons for non-participation included fear associated with taking medication (23.1%) and lack of time (16.9%). Conclusions: Postmenopausal women participate in clinical trials to help others and potentially themselves. Barriers to participation in trials may include the intervention being evaluated and time required to participate in the trial. Researchers should consider these motivations and barriers when recruiting postmenopausal women for RCTs.