Background: Outcome monitoring of depression is recommended but lacks evidence of patient benefit in primary care. Aim: To test monitoring depression using the PHQ-9 questionnaire with patient feedback. Design and setting: Open cluster-randomised controlled trial in 141 group practices. Method: Adults with new depressive episodes were recruited through records searches and opportunistically. Exclusion criteria: dementia, psychosis, substance misuse, suicide risk. The PHQ-9 questionnaire was to be administered soon after diagnosis, and 10-35 days later. Primary outcome: Beck Depression Inventory (BDI-II) score at 12 weeks. Secondary outcomes: BDI-II at 26 weeks; Work and Social Adjustment Scale and EuroQol EQ-5D-5L quality of life at 12 and 26 weeks; antidepressant treatment, mental health service use, adverse events, and Medical Informant Satisfaction Scale over 26 weeks. Results: 302 intervention arm patients were recruited and 227 controls. At 12 weeks 252 (83.4%) and 195 (85.9%) were followed-up respectively. Only 41% of intervention arm patients had a GP follow-up PHQ-9 recorded. There was no significant difference in BDI-II score at 12 weeks (mean difference -0.46; 95% CI -2.16,1.26), adjusted for baseline depression, baseline anxiety, sociodemographic factors, and clustering by practice). EQ-5D-5L quality of life scores were higher in the intervention arm at 26 weeks (adjusted mean difference 0.053; 95% CI 0.093,0.013). A clinically significant difference in depression at 26 weeks could not be ruled out. No significant differences were found in social functioning, adverse events, or satisfaction. In a per-protocol analysis, antidepressant use and mental health contacts were significantly greater in intervention arm patients with a recorded follow-up PHQ-9. Conclusions: No evidence was found of improved depression outcome at 12 weeks from monitoring. The findings of possible benefits over 26 weeks warrant replication, investigating possible mechanisms, preferably with automated delivery of monitoring and more instructive feedback