A total of 56 human bladder tumors that were histologically proved to be transitional cell carcinoma was analyzed by simultaneous flow cytometric 2-color measurements of deoxyribonucleic acid (DNA) and bromodeoxyuridine. Bromodeoxyuridine in vitro labeling was performed by sample incubation with bromodeoxyuridine under high atmospheric pressure oxygen. Grade 1 tumors showed 33.3% aneuploidy with a mean bromodeoxyuridine positive stained ratio (labeling index) of 5.1 +/- 3.4%. Grade 2 tumors featured 51.7% aneuploidy with a mean labeling index of 8.9 +/- 7.7%. On the other hand, a markedly increased labeling index of 15.2 +/- 8.2% with aneuploidy was observed in all but 1 grade 3 tumors. When DNA ploidy and labeling indexes were compared according to the presence or absence of muscular invasion of tumors, all 16 muscle invasive tumors showed aneuploidy (mean labeling index 18.7 +/- 8.0%), while 17 of 40 nonmuscle invasive tumors showed aneuploidy (mean labeling index 8.6 +/- 5.4%). This labeling index difference was statistically significant (p less than 0.01). These results indicate that bromodeoxyuridine/DNA 2-color flow cytometry may provide an objective parameter for quantification of the malignant potential of bladder cancers.