OBJECTIVES: Inadequate adaptation to long-term treatment of chronic illnesses is the most common reason for the inability to obtain the benefits medications can provide. Treatment compliance is influenced by several factors. Beliefs about Medicines Questionnaire (BMQ) developed by Horne et al. in 1999 to evaluate the cognitive representation of medicines have many validation studies, which resulted in good psychometric properties. The aim of the present study was to evaluate the reliability and validity of the BMQ Turkish translation in patients with asthma and chronic obstructive pulmonary disease (COPD).