Using the flow-cytofluorometric technique, 47 consecutive cases of pituitary tumors 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 tumor were analyzed. Tumors causing acromegaly showed a higher incidence of high percentages of S-phase cells (.gtoreq. 10%) than any other endocrinological type of pituitary tumor. Three of 6 tumors with high percentages of S-phase cells showed clinically aggressive growth patterns.