OBJECTIVES Low adherence to treatment with bisphosphonates significantly impedes its effectiveness. The objectives were: (1) to compare adherence to oral weekly and monthly bisphosphonates with emphasis on dosing instructions; and (2) to study associations between adherence and beliefs about the bisphosphonate treatment among women ≥ 55 years. METHODS A multicenter survey was performed in secondary-care patients with osteoporosis. Osteoporosis Specific Morisky Medication Adherence Scale (OS-MMAS), questions on compliance with five dosing instructions and Beliefs about Medicines Questionnaire (BMQ) Specific were used. RESULTS As many as 363 questionnaires (response rate 95%) were analyzed. Respondents (mean age 69 years) were treated with weekly bisphosphonates (37%) or monthly ibandronate (63%). Based on OS-MMAS, 67% of respondents showed high adherence with no differences between the subgroups. Only 44% of respondents were compliant with all dosing instructions. Compliance with dosing instructions concerning time interval (fasting and staying upright) was 71% in weekly and 52% in monthly subgroups, respectively (p < 0.001). Compliance with dosing instructions correlated positively with education (p = 0.009). The mean BMQ necessity score of 18.4 was greater than the mean BMQ concerns score of 13.3. OS-MMAS score correlated with necessity (p = 0.010). Persistence derived from OS-MMAS correlated with both necessity (p = 0.014) and concerns (p = 0.041). CONCLUSION Despite relatively high adherence to the treatment, most patients do not follow dosing instructions. Reduced bioavailability, particularly of monthly ibandronate, can be expected in clinical practice. Adherence-related outcomes are associated with beliefs about the oral treatment with bisphosphonates.