BACKGROUND: MACRO (Defining best Management for Adults with Chronic RhinOsinusitis) is an NIHR-funded programme of work designed to establish best practice for adults with chronic rhinosinusitis (CRS). The 7-year programme comprises three consecutive workstreams, designed to explore NHS care pathways through analysis of primary and secondary data sources, and to undertake a randomised controlled trial to evaluate a longer-term course of macrolide antibiotics and endoscopic sinus surgery for patients with CRS. A number of outstanding elements still required clarification at the funding stage. This paper reports an expert panel review process designed to agree and finalise the MACRO trial design, ensuring relevance to patients and clinicians whilst maximising trial recruitment and retention. METHODS: An expert panel, consisting of the MACRO Programme Management Group, independent advisors, and patient contributors, was convened to review current evidence and the mixed-method data collected as part of the programme, and reach agreement on MACRO trial design. Specifically, agreement was sought for selection of macrolide antibiotic, use of orally administered steroids, inclusion of CRS phenotypes (with/without nasal polyps), and overall trial design. RESULTS: A 12-week course of clarithromycin was agreed as the main trial comparator due to its increasing use as a first- and second-line treatment for patients with CRS, and the perceived need to establish its role in CRS management. Orally administered steroids will be used as a rescue medication during the trial, rather than routinely either pre or post trial randomisation, to limit any potential effects on surgical outcomes and better reflect current UK prescribing habits. Both CRS phenotypes will be included in a single trial to ensure that the MACRO trial is both pragmatic and generalisable to primary care. A modified, three-arm trial design was agreed after intense discussions and further exploratory work. Inclusion criteria were amended to ensure that the patients recruited would be considered eligible for the treatment offered in the trial due to having already received appropriate medical therapy as deemed suitable by their ENT surgeon. A proposed 6-week run-in period prior to randomisation was removed due to the new criteria prior to randomisation. CONCLUSION: The expert panel review process resulted in agreement on key elements and an optimal design for the MACRO trial, considered most likely to be successful in terms of both recruitment potential and ability to establish best management of patients with CRS.