The authors evaluate the accuracy and practical applications of flow cytometry (FCM) on bladder lavage fluid in the diagnosis and follow-up of bladder tumours. The apparatus used was a Coultronics Epics Profile II cytofluorograph. Two hundred and fifteen samples were obtained with a yield of 86%. The specimens were preserved in ethanol. The staining was performed on whole cells with preservation of the cytoplasm. The analysis of a control group of 45 patients confirmed that the FCM study of ploidy was specific (0.97 for normal bladders and 0.8 for inflammatory lesions). Four aspects were evaluated: Correlation between FCM and the histological type of the tumour: a significant difference was observed between the control group, the invasive tumour group (p < 0.01) and the carcinoma in situ group (< 0.001). A significant difference was observed in the case of high-grade PTA and PT1 tumours. No significant difference was observed between FCM and classical cytodiagnosis when this technique was performed by a trained cytologist. Predictive value of FCM for the recurrence of PTA and PT1 tumours: 40 patients were followed with a mean follow-up of 13 months. The relative risk of recurrence in the case of a tumour with an abnormal FCM was 2 (p < 0.05). FCM and monitoring of conservatively treated tumours: 30 patients with normal endoscopic examination after endoscopic resection of a PTA or PT1 tumour underwent cytometric analysis and cytodiagnosis. In the case of an abnormality on cytometry, randomised bladder biopsies and urography were performed. The positive predictive value for the presence of a lesion not diagnosed by cystoscopy and detected by FCM was 0.38 +/- 0.26. FCM and intravesical chemotherapy: 16 patients with PT1 tumours and abnormal FCM received BCG therapy (11 patients) or mitomycin C instillations (5 patients). A significant difference (p < 0.01) was observed between the 2 treatments in terms of normalisation of cytometry.(ABSTRACT TRUNCATED AT 400 WORDS)