Deoxyribonucleic acid (DNA) flow cytometry measurements were performed in nuclear suspensions obtained from paraffin-embedded biopsies from 83 patients with stages T2, T3 and T4a bladder carcinoma. All patients were treated with preoperative radiotherapy and cystectomy from 1976 through 1985. Of the tumors 13 (16%) were diploid, 18 (22%) tetraploid and 52 (63%) aneuploid. A total of 19 tumors (23%) had 2 or 3 stemlines in addition to the diploid cells. Post-radiotherapy stage reduction (absence of muscle infiltration in the cystectomy specimen) occurred more often in tumors with only 1 nondiploid stemline than in diploid tumors or nondiploid tumors with multiple stemlines. The 5-year survival rate was significantly poorer for patients with a diploid (33%) than for those with a nondiploid (66%) tumor (p = 0.05), although this was only marginally retained in a multivariate analysis (p = 0.11). The clinical significance of DNA ploidy in muscle infiltrating bladder cancer seems not to be as evident as has been shown for superficial bladder tumors but it may be of value in selecting patients for preoperative radiotherapy.