Urine specimens from 77 patients with transitional cell tumors of the urinary bladder, 15 patients with inflammatory lesions and five normal individuals were analyzed. The two nuclepore slides of all patients with neoplastic disease contained over 100 neoplastic cells and in one-half of the cases over 1,000 tumor cells were present. The percentage of neoplastic cells to atypical and normal cells ranged from 25 per cent for papillary carcinomas, Grade I, to 36 per cent for poorly differenciated carcinomas. There were no significant differences of the cytoplasmic features but the nuclei of malignant cells were slightly more hyperchromatic, contained significantly more nuclei and were characterized by a coarsely rather than finely granular nucleoplasm. The cell size of 5,970 cells and the nuclear size of 8,455 cells was determined by planimetry. Normal transitional cells averaged 341 square microns and were larger than those exfoliated from tumors. The mean nuclear area was 36 square microns for normal cells, 52 square microns for benign atypical cells, 54 square microns for cells from papillary carcinoma, Grade I, 78 square microns for papillary carcinomas, Grade II, and 90 square microns for transitional cell carcinoma, Grades III and IV. The study indicated that significant quantitative differences exist between normal transitional cells and those exfoliated from moderately and poorly differentiated transitional cell carcinomas. The cells exfoliated from papillomas or papillary transitional cell carcinomas, Grade I, closely resemble hyperplastic or atypical transitional cells which in turn can be differentiated from normal urothelial cells.