Immunoreactivity and receptor expression of insulinlike growth factor I and insulin in human adrenal tumors. An immunohistochemical study of 94 cases. Using immunoperoxidase methods, 94 human adrenal tumors were examined for evidence of immunoreactivity and receptor expression of insulinlike growth factor I (IGF-I) and insulin. The frequency of IGF-I in adrenocortical carcinomas was significantly higher than that in adenomas of the adrenal glands. The adrenocortical carcinomas showed strong intensity of staining for IGF-I, IGF-I receptors, and insulin receptors. A significant correlation between immunoreactivity and receptor expression of both IGF-I and insulin was found only in the adrenocortical carcinomas. The adrenocortical adenomas with Cushing's syndrome and pheochromocytomas, more than adrenocortical adenomas with Conn's syndrome, also stained strongly for insulin receptors. Thus the IGF-I and insulin probably play a role in the growth of adrenocortical carcinoma tissues, possibly through autocrine mechanisms. The expression of insulin receptors in adrenocortical adenomas in the presence of Cushing's syndrome and pheochromocytomas may be associated with functions. 