BACKGROUND: Primary care guidelines recommend GPs consider using depression symptom questionnaires as patient-reported outcome measures (PROMs) to monitor depression in adults to inform treatment and evaluate management strategies. The PROMDEP randomised controlled trial assessed the use of the Patient Health Questionnaire's (PHQ-9) effectiveness and cost-effectiveness for monitoring depression. We gathered qualitative evidence on the views and experiences of participating patients and practitioners to inform interpretation of the findings. AIM: To explore the views and experiences of patients and practitioners of using the PHQ-9 in the PROMDEP trial of monitoring depression in primary care. DESIGN & SETTING: Nested qualitative study and process evaluation of the trial in primary care in England and Wales. METHOD: Twenty-nine patients and 15 practitioners took part in semi-structured telephone or video interviews. Interview data were analysed using thematic analysis. RESULTS: Patients and practitioners both valued the PHQ-9 and reported limitations in its use for monitoring depression. This included its role in improving understanding of depression, impact on consultation and care, and integration within current primary care processes. In the context of the PROMDEP trial, our findings highlight potential reasons for the mixed trial findings, including how resistance in its use in practice may be owing to barriers that make it hard for practitioners to integrate the PHQ-9 in consultations. CONCLUSION: Monitoring of depression using PROMs needs to be considered within the context of current primary care processes and resources. Further research is warranted to understand how the PHQ-9 can be successfully integrated into consultations.