Flow cytometric deoxyribonucleic acid measurements were performed on 26 adrenocortical tumours and 9 non-tumours adrenals. All but one tumours were classified both histologically and clinically as benign, however, two thirds of them had abnormal deoxyribonucleic acid stemlines. Proliferative indices of adenomatous tissues were significantly higher than those of non-tumorous adrenals (p < 0.01). When compared to tumors smaller than 5 cm in size, tumours larger than 5 cm displayed significantly higher proliferative indices (p < 0.01). Thus, flow cytometry appears to have only a limited value in distinguishing between benign and malignant adrenocortical tumours, but it may provide additional information about the prognosis of these tumours.