1,900 cytological analyses of urine and bladder washings were made in 127 patients with urothelial bladder carcinomas before, during and after therapy. Following transurethral resection, all patients were treated by intravesical instillation of mitomycin C or thiotepa. Because of a locally advanced bladder cardinoma, 26 patients who were not candidates for radical cystectomy were given an integrated treatment of radiotherapy and chemotherapy. Intravesical administration of mitomycin C and thiotepa as well as integrated radiotherapy and chemotherapy induce a variety of cytological effects (toxic and/or metabolical) which may lead to cytological misinterpretations in the follow-up. DNA measurements by means of single-cell spectrocytophotometry show that the cytological effects induced by the above-mentioned therapies are not accompanied by an increase in the nuclear DNA content. It is concluded that the knowledge of these induced effects is mandatory for a correct interpretation of urinary cytology in the follow-up. Considering these effects and the clinical history, bladder carcinoma recurrences during and after intravesical chemotherapy or integrated radiotherapy and chemotherapy may be detected early by urinary cytology in the hands of an experienced cytopathologist or urologist. Furthermore, alterations of the urinary cytology occur after systemic application of cyclophosphamide and under immuno-suppressive therapy.