Objective: Individuals who are adherent to their asthma inhaled maintenance medication: (1) initiate their treatment (take the first dose); (2) implement it (take it as per prescribed dosing regimen); and persist with it (take it for the entire prescribing period). To avoid unnecessary hospitalizations and costs, patient adherence should be routinely assessed. To this end, we aimed to identify patient-reported outcome instruments (PROs) used for measurement of patient adherence to inhaled maintenance medication in asthmatic adults and to report on the adherence stage that these instruments measured. Data sources: We conducted a scoping review of six databases. Study selection: We searched for studies in which PROs were used to measure patient adherence to inhaled maintenance medication in asthmatic participants aged ≥18 years. We extracted and synthesized data in order to list the available PROs and to report the adherence stages that these instruments measured. Results: We included 186 studies (87 PROs). Among all 87 instruments, none were found to measure all three adherence stages. We found that 1 measured initiation, 2 evaluated initiation and implementation, 74 assessed implementation, 7 measured implementation and persistence. The most used instrument (n = 41 studies or 22%) was the Self-Reported Medication-Taking Scale, which was found to assess implementation only. Conclusion: Our results suggest that no single PRO exists to measure initiation, implementation and persistence with asthma inhaled maintenance medication. Results from this review could therefore guide the development or refinement of PROs that would aim to measure all three adherence stages.