Ad4BP, a zinc finger DNA-binding protein, is a transcription factor that regulates the expression of the steroidogenic P450 genes. We performed immunoblotting and immunohistochemistry of Ad4BP in 34 human adrenal cortex specimens, which included adrenocortical adenomas and carcinomas. Immunoblotting revealed a single hand of 53K, corresponding to the mol wt of Ad4BP. The immunohistochemical studies demonstrated that Ad4BP immunoreactivity was present exclusively in the nuclei of nearly all of the adrenocortical parenchymal cells in both the normal and the pathological human adrenal specimens. Ad4BP was immunostained with equal intensity and frequency among the different cell types. Ad4BP immunoreactivity was also observed in areas of marked degenerative changes, such as lipomyelomatous lesions, and in poorly differentiated carcinoma cells. These results suggest a close association of Ad4BP expression with the biological phenotype of adrenocortical parenchymal cells. Ad4BP therefore seems to play important roles in the induction and maintenance of the transcription of all steroidogenic P450 genes in human adrenocortical cells, even after malignant transformation.