IGF-I is one of the peptides that participates in normal adrenocortical cell growth and, possibly, in the genesis and/or maintenance of tumors and hyperplasias of the adrenal cortex. An immunohistochemical technique was used for the analysis of IGF-l expression in eight control and four hyperplastic adrenals, 11 adrenal cortical adenomas, and 18 adrenal cortical carcinomas. A large number of IGF-I positive cells with granular cytoplasmic (GC) staining pattern was found in the reticularis layer of control adrenal tissues. Fifty percent of the hyperplasias had the GC pattern and the other 50% a mixed pattern; in 64% of the tumors, the adenomas showed a LM (linear membrane) pattern, while adenocarcinomas usually showed a GC pattern (94%). Approximately 75% of the hyperplasias had between 10 and 50% of IGF-l positive cells, while adenomas and carcinomas had over 50% of IGF-I positive cells in 64% and 83% of the samples, respectively. The size of the tumors with 50% positive cells, compared with those with less than 50%, was, on average, greater, but no statistical differences between cell positivity and corresponding clinical syndrome were observed. Thus, detection of IGF-l in control and pathological adrenal tissue suggests participation of this growth factor in cell function and/or growth and proliferation.