From 1976 through 1983, 48 patients with T2/T3 bladder carcinomas were treated with pre-operative radiotherapy (46 Gy) and total cystectomy. DNA histograms were recorded by flow cytometry (FCM) from all pre-treatment paraffin-embedded tumour biopsies and, if still present after radiotherapy, from tumour tissue from the cystectomy specimen. Five patients were excluded from the study because no DNA histograms could be recorded due to extensive destruction of the cell nuclei during preparation. Thus, 43 patients are completely evaluable. Before the start of radiotherapy, 32 bladder carcinomas were interpreted as non-diploid, whereas 11 were diploid. Non-diploidy of the pre-treatment biopsy was associated with radiotherapy-induced stage reduction (p = 0.13). The survival rates in patients with diploid and non-diploid tumours were 65% and 45% respectively at 4 years (p = 0.13). Although not statistically significant, our results suggest that DNA-FCM may provide clinically relevant information about the tumour biology and response to radiotherapy concerning bladder carcinomas.