\ua9 2025 The Author(s). Published by Oxford University Press on behalf of European Society of Endocrinology.Objective: Surveillance is often adopted for asymptomatic non-functioning pituitary macroadenomas (macroNFPAs). Due to low-quality evidence, uncertainty remains on optimal frequency of imaging/biochemical monitoring and indications for surgery. We assessed the natural history and outcomes of patients with macroNFPA who had monitoring as initial management choice from the UK NFPA Consortium. Design: This was a multicentre, retrospective, cohort study involving 21 UK endocrine departments. Methods: Clinical, imaging, and hormonal data of 949 patients followed up between January, 1, 2005 and March, 1, 2022 were analysed. Results: Incidence rate for tumour enlargement was 9.8 per 100 patient-years (95% CI, 8.8-10.8), with cumulative probabilities 1.6%, 8.1%, 18.4%, 29.2%, and 43.6% at 6-month, 1-year, 2-year, 3-year, and 5-year follow-up, respectively; rates were higher in tumours abutting/displacing optic chiasm than those not in contact with it. Amongst macroNFPAs not in contact with optic chiasm showing enlargement within 6 months, none impacted visual fields. In tumours with enlargement and continued monitoring (median 2.6 years), further growth occurred in 60.5% (33.8% probability at 2 years), stability in 35.5%, and shrinkage in 4.0%. Rates of new pituitary hormone deficits were 4.0%-4.9%, mainly driven by tumour enlargement. After transsphenoidal surgery, rates of hypopituitarism reversal were 12%-17% and those of additional anterior pituitary hormone deficits were 12%-15% (permanent vasopressin deficiency 3.5%). Conclusions: Our data provide evidence for monitoring protocols. MacroNFPAs not in contact with optic chiasm require less frequent imaging, and first follow-up scan can be delayed to 1 year. After first enlargement, variable tumour behaviour can occur. New hypopituitarism in stable tumours is rare, challenging necessity of regular pituitary function assessment
