Analysis of cellular deoxyribonucleic acid content via flow cytometry was done on tumor biopsies and bladder washings from 61 patients with transitional cell carcinoma who underwent preoperative irradiation (36 or 20 Gy) and cystectomy. Classification only of tumors as diploid or various forms of aneuploid did not facilitate the separation of these advanced tumors into different subgroups. In addition, the proliferation rates, which also were high, did not facilitate further subclassification. On the other hand, more than 1 aneuploid cell line in the preoperative biopsies was linked more closely to pathological than to clinical stage and was found particularly if the stage was unchanged or increased after irradiation. The deoxyribonucleic acid pattern generally was the same after irradiation in patients with residual tumor and persistent aneuploidy. However, a diploid deoxyribonucleic acid value was noted in a number of patients with residual tumors, which indicated the existence of diploid and aneuploid tumor cells, and a preferential irradiation effect on the aneuploid portion of the tumor cell population.