Using the flow-cytofluorometric technique, 47 consecutive cases of pituitary tumours were analyzed regarding DNA ploidy and the percentage of cells in different phases of the cell cycle. About 20,000–30,000 cells from each tumour were analyzed. Tumours causing acromegaly showed a higher incidence of high percentages of S-phase cells (≧ 10%) than any other endocrinological type of pituitary tumour. 3 of 6 tumours with high percentages of S-phase cells showed clinically aggressive growth patte