The DNA ploidy of bladder cancers treated by radical cystectomy following pre-operative irradiation was analyzed by flow cytometry using paraffin embedded samples. The DNA ploidy and its changes by irradiation were studied. We used flow cytometry in 30 patients with transitional cell carcinoma of the bladder who received pre-operative irradiation (40 Gy in 24 patients, 20 Gy in 5 patients and 60 Gy in one) with follow-up for at least 3 years. Total 140 paraffin embedded samples (4.6 samples per one patient) were available. The effects of therapy were related to the DNA patterns before irradiation and to the DNA ploidy changes after irradiation. 1. Eight DNA diploid tumors and twenty-two DNA aneuploid ones were detected before irradiation. Although diploid group didn't change its DNA ploidy after irradiation, of 22 aneuploid tumors 18 were changed to DNA diploid and 4 were not changed in their ploidy. 2. The tumor eradicating effect of irradiation was shown to be higher (p < 0.05) in the diploid group (5 of 8, 63%) than in the aneuploid group (5 of 22, 23%). 3. Overall survival rates were discussed in 3 groups (A, B and C), the group A was 10 of tumor free and 3 diploid tumors after irradiation, the group B was 13 of aneuploid tumors which changed to diploid ones and the group C was 4 of persistent aneuploid tumors. Each of 5 year survival rate was 100% (A), 58% (B) and 0% (C). Overall survival for C group was significantly shorter than for other groups (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)