Tumor deoxyribonucleic acid (DNA) ploidy was evaluated as an objective parameter that may correlate better with the responsiveness of bladder cancer to chemotherapy plus radiotherapy than do clinical features or histopathological subtypes. A total of 40 patients with localized muscle-invading bladder cancer (clinical stages T2 to T4) underwent prospective treatment on a potential bladder preserving protocol. Tumors of 37 of the 40 patients were analyzed by DNA flow cytometry of multiple paraffin embedded specimens. Transurethral resection, neoadjuvant chemotherapy and 40 Gy. radiotherapy plus cisplatin were followed by urological reevaluation of the tumor (a complete response required a negative biopsy and a negative urine cytology study). A total of 7 noncomplete response patients underwent immediate radical cystectomy whereas the full bladder sparing treatment (radiotherapy to 64.80 Gy. plus cisplatin) was given to 23 complete response patients and 7 noncomplete response patients who were unsuited for surgery. Of the tumors 22 (59%) were purely aneuploid and 10 (27%) were purely diploid. Five tumors contained aneuploid and diploid patterns in different tumor specimens (partly diploid). Current status with a 30-month median followup in surviving patients includes an 82% overall survival rate in the aneuploid group versus 47% in the diploid/partly diploid group. Of all the patients 68% are free of invasive tumor: 82% in the aneuploid group versus 47% in the diploid/partly diploid group. By multivariate analysis pure aneuploidy was significantly (p = 0.05) correlated with freedom from invasive tumor in the bladder (either as persistence or as recurrence) and approached significance (p = 0.08) in correlation with overall patient survival. A longer observation time will be required to confirm this unexpectedly good outcome for patients with pure aneuploid tumors. We hypothesize that pretreatment DNA ploidy status may become a clinically useful prognostic factor in selecting patients for successful treatment with transurethral surgery and neoadjuvant chemotherapy plus radiotherapy.