30% of meningiomas are incidental findings. There is no consensus on the optimal management (active monitoring, surgery, radiosurgery). Develop a prognostic index to identify patients at risk of active monitoring failure. Active monitoring failure was defined as: new symptoms, MRI progression (absolute growth rate 2 cm3/year or absolute growth rate 1 cm3/year + relative growth rate 30%/year) or loss of treatment options. A prognostic model was developed using MRI and patient co-morbidity (Charlson-Index) in a retrospective cohort (2007–2015). 385 patients (403 meningiomas) were studied; mean age was 62.6 years (SD=12.0); 301 (78.2%) were female. Over a median of 36.0 months (range: 3–120), 1688 MRI were performed (mean=4 scans/patient). 44 (10.9%) meningiomas failed active monitoring. Median time to failure was 33.0 months (range: 5–102). Model parameters were based on statistical and clinical considerations and included: increasing tumour volume (HR=2.17 [95% CI=1.53–3.09], p