PURPOSE A decision tree in diagnosis of incidentally detected adrenal masses (incidentaloma) was made on the basis of these results. METHODS The clinical usefulness of adrenal scintigraphy with 131I-adosterol and ultrasonically guided tumor biopsy was investigated in 44 patients. RESULTS Adrenal scintigraphy was performed in 32 patients, of whom 21 were found to have an increased uptake in the tumor, including 19 cases of cortical adenoma and 2 of hematoma. No abnormal uptake was found in the remaining 11 patients, including 2 of cortical adenoma, 1 of adrenocortical oncocytoma and 8 of non-cortical tumors. Adrenal scintigraphy was thus thought to be useful in the differentiation of cortical tumor from non-cortical tumor, showing the sensitivity of 86%, the specificity of 80% and the diagnostic accuracy of 84%. Cytological or histological study on specimens obtained by percutaneous adrenal tumor biopsy was performed in 19 patients, of whom 18 (95%) were correctly diagnosed in terms of the malignancy of incidentaloma. CONCLUSIONS Taken together, the differential diagnosis and the surgical indication of adrenal incidentaloma could be made successfully based on adrenal scintigraphy and tumor biopsy.