Nineteen patients with newly diagnosed transitional cell carcinoma of the bladder consented to receive an intravenous bolus of bromodeoxyuridine (BrdUrd) 3 to 6 h prior to transurethral resection of bladder tumour. Routine paraffin embedded sections were stained with anti-BrdUrd monoclonal antibody in order to determine the S phase fraction of the bladder tumours and to correlate this with tumour grade, category and the development of recurrence during follow-up. BrdUrd labelling of nuclei was reliably detected in 100% of the tumours. The mean labelling index of Ta tumours (n = 8) was 3.14%; in T1 tumours (n = 7) it was 8.6% and in T2-3 tumours (n = 4) it was 16.4%. A similar correlation was found in association with tumour grade. The patients were followed up for a mean of 23 months (range 18-30). Seven of the 15 superficial tumours recurred during the period of follow-up. These tumours had a mean labelling index (at diagnosis) of 8.4%, whereas the tumours that did not recur had a mean labelling index of 3.4%. The in vivo determination of BrdUrd uptake in transitional cell carcinoma of the bladder may be useful in predicting which superficial tumours are likely to recur following resection and so rationalise both the time to follow-up and intravesical chemoprophylaxis.