A total of 98 urologic outpatients with a history of conservatively treated low stage bladder tumors underwent 325 examinations during a 2-year interval, each comprising conventional cytology, cystoscopy and automated flow cytometry. The flow cytometry results agreed with cystoscopic findings and conventional cytology in 80 per cent of the examinations, while cystoscopy and cytology agreed in 75 per cent. Assuming cystoscopic findings were entirely accurate, the false negative rate for flow cytometry was 8.3 per cent and the false positive rate was 5.8 per cent. On occasion, flow cytometry diagnosis of bladder tumors antedated the development of visible tumor by up to 12 months. Flow cytometry provided an objective and quantitative assay of epithelial abnormalities and, in the absence of skilled, conventional cytology, it appears to be a valuable new tool for the urologist in the diagnosis, management and followup of patients with bladder tumors or a history of bladder tumor.