Few data are available on physician perceptions of osteoporosis medication adherence. This study compared physician-estimated medication adherence with adherence calculated from their patients' pharmacy claims. Women aged ≥ 45 years, with an osteoporosis-related pharmacy claim between January 1, 2005 and August 31, 2008, and continuous coverage for ≥ 12 months before and after first (index) claim, were identified from a commercial health plan population. Prescribing physicians treating ≥ 5 of these patients were invited to complete a survey on their perception of medication adherence and factors affecting adherence in their patients. Pharmacy claims-based medication possession ratio (MPR) was calculated for the 12-month post-index period for each patient. Physicians who overestimated the percentage of adherent (MPR ≥0.8) patients by ≥ 10 points were considered "optimistic". Logistic regression assessed physician characteristics associated with optimistic perception of adherence. A total of 376 (17.2%) physicians responded to the survey; 62.0% were male, 58.2% were aged 45 to 60. years, 55.3% had ≥ 20 years of practice, and 35.4% practiced in an academic setting. Participating physicians prescribed osteoporosis medications for 2748 patients with claims data (mean [SD] age of 62.0 [10.6] years). On average, physicians estimated 67.2% of their patients to be adherent; however, only 40% of patients were actually adherent based on pharmacy data. Optimistic physicians (73.4%) estimated 71.9% of patients to be adherent while only 32.2% of their patients were adherent based on claims data. Physicians in academic settings were more likely to be optimistic than community-based physicians (odds ratio 1.69, 95% CI: 1.01, 2.85). Overestimation of medication adherence may impede physicians' ability to provide high quality care for their osteoporosis patients. © 2013 Elsevier Inc.