AIMS--To analyse the diagnostic value of cytological examination compared with histological findings in a large series of patients (n = 615) with tumours of the urinary tract epithelium. METHODS--Cytological examinations (n = 785) after bladder washing and exfoliative cytology were retrospectively compared and correlated with histological findings. In addition, 1527 bladder washings were obtained during follow up of patients after transurethral resection of bladder tumours. RESULTS--Cytology in bladder washings (overall diagnostic accuracy 66%) provides considerably more information that exfoliative cytology (overall accuracy 49%). Cytological examinations (n = 1125) in patients with bladder tumours receiving intravesical cytostatic drugs (for example, mitomycin C) yielded suspicious or positive results in 28% of patients, without being confirmed by endoscopy during follow up. CONCLUSION--Our results illustrate two major drawbacks of urinary cytology. First, a high rate of false positive results in patients on intravesical chemotherapy. Second, a high rate of false negative results in highly differentiated carcinomas, stressing the need for additional diagnostic tests such as staining with monoclonal antibodies directed against tumour antigens or assessment of ploidy.