The DNA content of 72 bladder tumors and 20 healthy bladders was studied with flow cytometry. Reproducibility of DNA measurement was systematically verified with the use of 3 controls: calf thymocytes, mouse hepatocytes and a mixture of human lymphocytes with trout red blood cells. For each tumor or normal bladder, a first sample containing urothelial cells only and a second sample with the addition of trout red blood cells were systematically examined. In the first group of patients with healthy bladders the percentage of urothelial cells with a DNA content greater than G0 G1 (2N) was on average 7.9% +/- 2.8%. Among the second group of 72 tumors, 2 DNA profiles were remarkable: in a first group (39/72; 55%) DNA profile was unimodal (single diploid peak); in a second group (33/72; 45%) DNA profile was bimodal (association of a diploid peak with an aneuploid one). Comparison of flow data with pathology data (WHO classification) clearly demonstrated that the presence of an aneuploid peak, characterizes grade elevation and infiltration. Hence, 1/16 (6%) of grade I; 7/75 (28%) of grade II; 25/31 (81%) of grade III tumors show an aneuploid peak, as well as 11/44 (25%) of Ta; 9/14 (64%) of T1; 5/6 (83%) of T2, and 8/8 (100%) of T3 tumors. Demonstration of an aneuploid peak with flow cytometry particularly in the group of patients having non infiltrating Ta tumors (11/44 in our series) could further either lead to a closer follow-up of this high risk group of patients or to the early recommendation of a non conservative treatment.