The presence of P-glycoprotein, associated with multiple drug resistance and present in the normal adrenal cortex, was studied in 15 cases of adrenocortical carcinoma. P-glycoprotein was found in eight tumours; no correlation was found with clinical presentation, steroid production or histological index. 10 patients received mitotane. Remarkably, 3 patients with P-glycoprotein-positive tumours achieved complete remission. On the other hand, 2 patients with P-glycoprotein-negative tumours showed progression of the disease despite mitotane treatment. These findings suggest that the expression of P-glycoprotein in adrenocortical carcinoma is not related to clinical manifestations, steroid production, histological index or response to mitotane therapy.