Current clinical teaching indicates that large (greater than 5 cm in diameter) adrenal masses are often malignant. In a retrospective analysis of patients studied between 1977 and 1988 with computed tomography (CT), adrenal scintigraphy, and when available, magnetic resonance imaging (MRI) 45 were found to have adrenal masses greater than 5 cm (range 5-19 cm) in diameter. Thirty were benign (16 phaeochromocytomas, six adrenocortical adenomas, four adrenal cysts, two myelolipomas, an adrenal hematoma and a ganglioneuroma). Of 15 malignant masses, there were seven adrenocortical carcinomas, five adrenal metastases and three adrenal lymphomas. With the exception of the adrenal myelolipomas, cysts, and the ganglioneuroma neither CT nor MRI demonstrated sufficient diagnostic specificity to distinguish benign from malignant lesions. Functional scintigraphy with 131I-6-beta-iodomethyl-19-norcholesterol for suspected adrenocortical lesions and 131I-metaiodobenzylguanidine for suspected phaeochromocytomas frequently provided useful information.