BACKGROUND: Chimeric Antigen Receptor (CAR) T-cell therapies are effective for treating haematological cancers but carry risks of toxicity and substantial symptom burden. Patient-reported outcomes (PROs) could significantly enhance clinical management for patients undergoing these treatments. However, guidance on selection of PRO measures for monitoring adverse event and quality of life after CAR T-cell therapy is limited. This study aimed to achieve consensus among patients and healthcare professionals on the selection of PRO measures for an electronic PRO (ePRO) system for CAR T-cell therapy clinical settings. METHODS: Two-round modified Delphi study (online survey and consensus meeting) conducted from December 2023 to January 2024 to select PRO measures for the ePRO system, guided by a conceptual framework with four measurement domains: symptom burden, impacts of cancer and CAR T-cell therapy, treatment tolerability, and health-related quality of life (HRQoL). Database searches (PubMed, ePROVIDE, COSMIN, and COMET) and licensing websites of cancer-specific PRO measures identified 113 PRO measures. Measures were pre-specified for treatment tolerability and HRQoL domains and concept mapping established conceptual coverage for the remaining domains. Seven PRO measures were shortlisted and prespecified inclusion thresholds and stopping criteria guided Delphi panel selection. Registration: ISRCTN11232653. FINDINGS: Nineteen participants (5 CAR T-cell patients, 14 healthcare professionals/researchers) recruited from a UK National Health Service (NHS) cellular therapy centre and professional networks took part in Round One (Delphi online survey). Shortlisted measures were rated for relevance, comprehensiveness, and ease of understanding for the symptom burden and impacts of cancer and CAR T-cell treatment domains. Consensus was achieved after Round One, precluding the requirement for Round 2 (consensus meeting). The Symptom Burden Questionnaire™ (SBQ™) and the Quality of Life in Adult Cancer Survivors (QLACS) were selected to represent the Symptom Burden and Impacts domains, respectively. These measures, EQ5D-5L, measuring HRQoL, and Functional Assessment of Chronic Illness Therapy-Item GP5 (FACT-GP5), single-item global indicator of cancer treatment tolerability, will be included in the ePRO system. INTERPRETATION: In the absence of guidance on PRO measure selection for CAR T-cell therapies, consensus-based methods represent an important step towards use of PROs with this clinical population. Modest sample size and representativeness of the patient subgroup are limitations of this study. FUNDING: This study is funded by the National Institute for Health and Care Research (NIHR) Blood and Transplant Research Unit in Precision Cellular Therapeutics (NIHR203339). The views expressed are those of the authors and not necessarily those of the NIHR, NHS Blood and Transplant, or the Department of Health and Social Care.