Purpose: To assess the risk of developing autonomous cortisol secretion (ACS) and tumour growth in non-functioning adrenal incidentalomas (NFAIs). Methods: Multicentre retrospective observational study of patients with NFAIs. ACS was defined as serum cortisol >1.8 µg/dL after 1 mg-dexamethasone suppression test (DST) without specific data on Cushing’s syndrome. Tumour growth was defined as an increase in maximum tumour diameter >20% from baseline; and of at least 5 mm. Results: Of 654 subjects with NFAIs included in the study, both tumour diameter and DST were re-evaluated during a follow-up longer than 12 months in 305 patients. After a median follow-up of 41.3 (IQR 24.7–63.1) months, 10.5% of NFAIs developed ACS. The risk for developing ACS was higher in patients with higher serum cortisol post-DST levels (HR 6.45 for each µg/dL, p = 0.001) at diagnosis. Significant tumour growth was observed in 5.2% of cases. The risk of tumour growth was higher in females (HR 10.7, p = 0.004). Conclusions: The frequency of re-evaluation with DST in NFAIs during the initial 5 years from diagnosis can probably be tailored to the serum cortisol post-DST level at presentation. Re-evaluation of NFAIs with imaging studies, on the other hand, seems unnecessary in most cases, particularly if the initial imaging demonstrates features specific to typical adenoma, given the low rate of significant tumour growth.