20 patients with primary carcinoma in situ of the urinary bladder were studied by flow cytofluorometric DNA analysis (FCM). Aneuploidy, i.e. abnormal DNA pattern, was found in all cases. As a rule the aneuploid cell populations were found in the tri-tetraploid region, corresponding to 70--92 chromosomes in the tumour cell nuclei. In seven cases more than one aneuploid cell line was found. All these seven cases showed progression in the tumour disease within 2 years. By FCM it was also possible to determine the proliferation rate of the tumours and the relative number of aneuploid cells in each tumour cell sample. In progressive cases high values of these two variables were found.