BACKGROUND The incidence of adrenal incidentalomas has sharply increased in recent decades and concurrent subtle endocrine abnormalities, or even subclinical conditions, have been identified. Nonetheless, data concerning possible changes in adrenal size and/or hormonal pattern during follow-up are still inadequate. OBJECTIVE To evaluate long-term morphological and functional evolution of adrenal incidentalomas after initial diagnosis and to identify possible risk factors for hormonal hyperactivity and mass enlargement. METHODS Sixty-four patients (34-79 years) were followed-up for 12-120 months (median 25.5 months). Initial computerized tomography scan showed a unilateral mass in 51 patients and bilateral lesions in 13 patients. Average mass diameter at diagnosis was 2.5+/-0.1 cm (range 1.0-4.0). Twelve patients had subclinical Cushing's syndrome, 41 had mild hormonal alterations, and 11 had normal adrenal function at baseline. All patients were investigated by morphological and functional evaluation 6 and 12 months after diagnosis, and then at 1-year intervals. RESULTS During follow-up, a mass size increase >/=1 cm was observed in 13 patients, and 18 developed further subtle endocrine alterations. Cumulative risk of developing endocrine abnormalities was 17% at 1 year, 29% at 2 years, and 47% at 5 years. The risk was higher in the first 2 years of follow-up if the initial tumor diameter was >or=3 cm. Overall, cumulative risk of mass enlargement was 6% at 1 year, 14% at 2 years, and 29% at 5 years, and it was greater in patients with normal adrenal function than in those with subtle hormonal abnormalities (P<0.05). One female subject showed a mass enlargement after 6 months of follow-up and was eventually diagnosed with non-Hodgkin's lymphoma. CONCLUSIONS Patients with an adrenal incidentaloma are at risk for tumor growth and development of hormonal alterations. The risk of adrenal malignancy, although not elevated, also indicates the need for long-term follow-up.