Objective: The objective of this study was to implement a brief intervention in asthma within a UK national pharmacy chain, and through the use of mystery customers, measure the extent of delivery of the intervention over a 4-month period. Methods: Mystery customers gathered information on which of the asthma intervention questions were asked, and any lifestyle advice that was given. Additional information was collected on the type of store they visited and the personnel who served them. Results: Mystery customer results indicate delivery rates of the intervention between 48.1 and 84.3% across the 4-month assessment period. This intervention was delivered by both pharmacists and healthcare staff, although the extent and content of delivery varied across the 4 months. Conclusion: The use of mystery customers has helped to assess the successful take up and delivery of this intervention. Based on the learning points gained throughout this study, future services can be designed for other condition areas. Practice implications: Each service that is designed needs to be simple to implement and deliver, and have the flexibility for pharmacists and staff to adapt the services to meet local requirements.