In 53 women with urothelial tumors of the bladder, renal pelvis and ureter of different in growth stages and grades of differentiation, a negative correlation among the value of the mitotic index and sex chromatin content was found. With a decrease in the number of nuclei containing sex chromatin bodies we have noted not only a proportional mitotic index increase but also a regular connected decrease of histological differentiation and a deeper invasion of the tumor. The significance of this simple test consists in the fact that one may judge the rate of tumor growth by the sex chromatin content in a small volume of biopsy material. A table of mitotic index values depending upon the quantity of nuclei with sex chromatin bodies in transitional cell tumors is presented.