Cytokeratin 18 expression in transitional cells of 76 bladder tumors (11 grade 1, 33 grade 2 and 32 grade 3) and 10 normal biopsies was quantified by flow cytometry after immunolabeling with the anti-cytokeratin 18 monoclonal antibody RGE 53. Combined cytometric analysis of deoxyribonucleic acid content and cytokeratin 18 expression was conducted. Of 76 tumor biopsies 38 had a unimodal deoxyribonucleic acid profile with a deoxyribonucleic acid index close to 1.0, while the other 38 showed a bimodal profile: 1 peak with an index of 1.0 and a second peak with an index of greater than 1.0. The 11 grade 1 tumors belonged to the first group, whereas 10 of the 33 grade 2 and 28 of the 32 grade 3 tumors belonged to the second group. Antibody RGE 53 reacted with 4 per cent of the normal bladder cells (umbrella cells), 14 +/- 3 per cent of the cells from grade 1, 33 +/- 8 per cent from grade 2 and 56 +/- 10 per cent from grade 3 tumors. Analysis of cytokeratin 18 expression according to deoxyribonucleic acid content at the single cell level in tumors with a bimodal deoxyribonucleic acid profile showed that cytokeratin 18 was detected frequently in cells with a high deoxyribonucleic acid index but much less so in cells with an index of 1.0. Therefore, expression of cytokeratin 18 can be recognized as a marker of aggressiveness in bladder tumors, since it increases in parallel with tumor grade and cell deoxyribonucleic acid content.