The DNA content of individual nuclei in bladder washings was estimated by means of flow cytometry. A method of preparation using a detergent to produce single nuclei was used. By means of a differential count the DNA distribution was corrected for the content of leucocytes. The DNA histograms were divided into a 2c fraction containing nuclei with less than 3c DNA, a 4c fraction with 3c—6c DNA and an 8c fraction with more than 6c DNA. Three groups of patients were investigated: 17 patients with invasive bladder tumours, 38 patients with non-invasive bladder tumours, and 22 patients with normal bladders. The washings from patients with invasive tumours contained significantly larger 4c and 8c fractions than washings from normal bladders. There was a correlation between cell anaplasia and the DNA distribution: grade III bladder tumours had larger 4c and 8c fractions than grade II tumours.