BACKGROUND: There are effective treatments to prevent osteoporotic fractures, but these treatments are underutilized. OBJECTIVE: To evaluate the influence of patient characteristics, perceptions, knowledge and beliefs about osteoporosis on the decision to initiate osteoporotic treatment. PARTICIPANTS: We identified female members of a managed care plan who had a dual energy x-ray absorptiometry (DXA) bone density test and fulfilled World Health Organization criteria for osteoporosis. Patients were excluded if they received osteoporotic medications in the prior 6 months. MEASUREMENTS: Patients were sent a questionnaire that included items assessing satisfaction with physician-patient communication, trust in the physician, osteoporosis knowledge and beliefs, beliefs about prescription medications, and perceptions of barriers to medication use. Administrative electronic health records were used to identify prescription drug use and health care utilization. RESULTS: Two hundred and thirty-six women returned surveys and research authorization forms out of 465 contacted for participation. One hundred and thirty-five (57.2%) filled a prescription for an osteoporotic drug in the first 3 months after the DXA exam. The largest differences between initiators and non-initiators were in beliefs in the benefits of medications, and distrust of medications, with initiators believing more strongly in the benefits and effectiveness of medications (p < .001), and non-initiators reporting more distrust of medications (p < .001). Osteoporosis knowledge and the belief that osteoporosis is a serious disease were also related to therapy initiation in bivariate analysis. CONCLUSIONS: Only 57% of patients initiated osteoporotic medication within 3 months of diagnosis. The decision to start osteoporosis treatment appeared to be related to a patient's beliefs in the effectiveness of osteoporosis medications and distrust of medications.