Background Insight into the delivery of interventions is necessary to gain a better understanding of what caused an intervention to succeed or fail. The Cardiovascular medication non-Adherence Tailored Intervention (CATI) study failed to show effectiveness of a patient-tailored, pharmacist-led intervention programme on self-reported adherence to antihypertensive medication. Objective To evaluate the implementation fidelity of the CATI intervention programme. Setting Twenty Dutch community pharmacies. Method The process of a randomised controlled trial was evaluated. Both quantitative and qualitative data were collected and analysed according to Carrolls' Conceptual Framework for Implementation Fidelity. Implementation fidelity is defined as the degree to which the intervention was implemented as intended. Main outcome measure Four key intervention components of the intervention programme (i.e., first consultation: barrier identification, information and advice, written summary, and follow-up consultation). Results For most participants the key intervention components were implemented as intended. The training of pharmacists, intensive monitoring during the study and structured and easy-to-use intervention materials facilitated the implementation of the intervention. The method to select participants for the intervention programme was considered insufficient and pharmacists questioned the eligibility of some participants because of a low degree of intake non-adherence. Conclusion Implementation fidelity was moderate to high for all key intervention components. Therefore, the absence of effectiveness of the CATI intervention programme on self-reported medication adherence cannot be explained by poor implementation of the intervention. However, the limited genuine eligibility of some participants resulted in a limited potential for improvement in medication adherence.