The in vivo labelling of urological tumour cells using the S phase marker bromodeoxyuridine (BRdU) for histochemical studies is reported. The use of multiparameter flow cytometry (FCM) with BRdU labelling to study tumour proliferation offers significant advantages. It provides simultaneous measurements of the DNA ploidy (DI), the duration of the S phase (Ts), the potential doubling time (Tpot) and the total and aneuploid tumour labelling indices (LI) from a single specimen. Heterogenous tumour cell populations can be measured with high sensitivity. We report a preliminary study to evaluate the method in the measurement of the kinetics of transitional cell carcinoma of the bladder (TCCB). Nineteen patients with TCCB, 1 with leukoplakia of the bladder, 2 with renal carcinoma, 1 with prostatic carcinoma and 1 with a squamous carcinoma of the penis were studied. Of the bladder tumours, 3 were aneuploid, DI = 1.32, 1.58 and 1.89. BRdU uptake was detected in all tumours. The median LI was 1.5% (range 0.5-10.0). In 15/19 tumours the labelling profile was satisfactory for calculation of the Ts and Tpot. The median Ts was 6.2 h and the median Tpot was 17.1 days. This study demonstrates that measurement of multiple parameters of urological tumour proliferation in vivo is possible. These parameters require further assessment as indices of biological aggressiveness and clinical prognosis.