Midstream urine samples were taken from 35 patients with histologically diagnosed urinary bladder carcinoma, from 27 patients without history or sign of malignancy or infection in the urinary tract, and from eight patiens with cystitis. The urine was filtered through Millipore filters, or Nuclepore filters, pore size 5.0 microns, on which the cells were caught. From the same patients biopsies were obtained from different anatomic regions of the urinary bladders as well as from the carcinomas that were present. The filters and the biopsies were examined with scanning electron microscopy (SEM). The exfoliated cells trapped on the filters were in good morphological condition. Idential characteristics of the cell surface morphology were recognized in the biopsies and the exfoliated cells from each patient. In non-malignant cases the cells displayed the regional morphological charcteristics of the urinary bladder. Bacteria and an increased number of red and white blood cells were attached to the cell surface of numerous exfoliated cells in patients with cystitis, but no pleomorphic microvilli were detected in these patients. In cases of malignancy exfoliated cells with pleomorphic microvilli on their surface were detected. A large majority of the exfoliated cells shed in the urine with pleomorphic microvilli on their surfaces still retained enough regional surface characteristics to determine their origin. Some of the cells were completely covered with pleomorphic microvilli and it was impossible to determine their origin.