To evaluate the risk of mass enlargement and endocrine function modification in patients with adrenal incidentaloma (AI). In this retrospective study, we examined clinical and hormonal characteristics of 310 patients with AI (200 females and 110 males; age: 58.3 ± 12.9 years), followed up for a median (interquartile range) of 31.4 months (13.0-78.6) and evaluated for possible modification in adrenal mass size and hormonal function. The hormonal evaluation included morning serum cortisol and plasma ACTH at 8 a.m., aldosterone, plasma renin activity/direct renin concentration, and 24-h urine metanephrines/normetanephrines. One microgram overnight dexamethasone suppression test (DST) was performed. Autonomous cortisol secretion (ACS) was diagnosed in the presence of cortisol after 1 mg DST > 5 μg/dl (138 nmol/l) or >1.8 and ≤5 μg/dl (50-138 nmol/l) and at least one of the following: (i) low ACTH; (ii) increased 24-h urinary-free cortisol; (iii) absence of cortisol rhythm; and (iv) post-LDDST cortisol level > 1.8 μg/dl (50 nmol/l). When there was no biochemical evidence of adrenal hormonal hyperactivity, AIs were classified as nonfunctioning (NFAIs). The mass was considered significantly enlarged when the size increase was more than 20% and at least 5 mm compared to baseline. At diagnosis, NFAIs were found in 209 patients, while ACS and overt adrenal hyperfunction were diagnosed in 81 and 20 patients, respectively. During follow-up, 3.3% and 1.5% of patients with NFAI developed subtle and overt endocrine hyperfunction, respectively, while a significant mass enlargement was observed in 17.7% of all AIs. The risk of developing ACS was significantly higher in patients with adenoma >28 mm (hazard ratio [HR] 12.4; 95% confidence interval [CI], 2.33-66.52, P = 0.003), in those with bilateral adrenal tumors (HR: 5.36; 95% CI, 1.17-24.48, P = 0.030), and with low/suppressed ACTH values (HR: 11.2, 95% CI 2.06-60.77; P = 0.005). The risk of mass enlargement was lower for patients in the fourth quartile of body mass index than those in the first quartile (HR 0.33; 95% CI, 0.14-0.78; P = 0.012). In patients with AI, the risk of developing hormonal hyperfunction and mass enlargement is overall low, although some tumor characteristics and anthropometric features might increase this risk. Taking account of all these aspects is important for planning a tailored follow-up in AI patients.