Background: Many patients experience difficulty taking medications resulting in suboptimal adherence. Ambiguity surrounding adherence issues in chronic illness has been exacerbated by a lack of patient-centered, medication-specific, and theoretically integrative measurement instruments. Aim: To develop a reliable and valid instrument to measure the factors that influence adherence in patients prescribed pharmacotherapy for coronary artery disease. Methods: Phase 1 involved the development of instrument structure and content. Constructs to be measured were defined through an analysis of adherence literature and qualitative interviews with patients. Phase 2 established the psychometric properties of the instrument. Exploratory factor analyses, reliability, and validity estimations were undertaken with a sample of patients (n 5 404) from 3 tertiary cardiology referral centers. Results: Factor analyses resulted in a logically coherent, 16-item, three-factor solution that explained 50.5% variance. The factors were labelled: "Medication Planning Strategies," "Health Risk, and Health Protection." Internal consistency reliability met acceptable standards (a 5 .700 to a 5 .785). Fair to excellent intraclass correlations for temporal stability were demonstrated (.498-.882). Preliminary construct validity was supported by promising findings in relation to content validity results and factor structure stability. Conclusions: A new adherence instrument for patients on pharmacotherapy for coronary artery disease has been developed and initial psychometric properties have been established. Additional instrument validation will be directed at further establishing construct and criterion-related validity. It is intended that this measure will be useful in identifying factors that impede or facilitate adherent behavior and contribute to advancing the science of instrument development within adherence research. © 2013 Springer Publishing Company.