A total of 316 biopsies from patients with bladder carcinoma who entered a trial of the genito-urinary group of the European Organization for Research and Treatment of Cancer (EORTC) were reviewed. The histological data were subsequently correlated with the clinical course. A strong correlation between the number of mitoses and the time of first recurrence, muscle invasion, and death was noted. Next the expression of Ki-67 in frozen sections from 49 transitional cell carcinomas (TCCs) and the DNA content of the tumour cells were determined. The frequency of Ki-67-positive tumour cells increased with tumour grade and stage. Grade II TCCs and superficially infiltrating TCCs showed a wide range of Ki-67 scores. There was a significant difference in Ki-67 score between non-infiltrating (Ta) and superficially infiltrating (T1) grade II TCCs. All DNA-aneuploid carcinomas but also 15 out of 36 DNA-diploid tumours contained more than 10 per cent Ki-67-positive cells. Only some of these tumours were DNA-aneuploid or -tetraploid. The results indicate that the number of Ki-67-positive cells in grade II tumours may be a useful aid in separating grade II TCCs with a favourable prognosis from those with a poor clinical outcome.