The potential use of numerical chromosomal abnormalities as predictive factors for the clinical behaviour of transitional cell carcinoma (TCC) was investigated. The effects on survival and progression-free survival were measured in 91 patients with TCC treated by transurethral resection. The survival rate of patients having tumours with a diploid chromosomal modal number was significantly better than that of patients having tumours with a hyperdiploid chromosomal modal number. The survival rate of patients having TCC with diploid cells only was also significantly better than that of patients having TCC with both diploid and hyperdiploid cells. Progression-free survival was significantly higher in patients having TCC with a diploid modal number of chromosomes than in patients with a hyperdiploid modal number. Simultaneous evaluation of the modal chromosome number or chromosomal range, histological grade, category and mitotic index of the tumour, and the patient's age and sex as prognostic factors in multivariate analyses showed that the category of bladder carcinomas was the most important factor in predicting the survival rate. In patients with superficial tumours (category Ta and T1) the modal chromosome number was the most important factor in predicting survival. For progression-free survival, the modal chromosome number appeared to be the most important factor. It was concluded that the modal chromosome number in TCC was useful in predicting survival in patients with superficial tumours and in predicting progression-free survival in patients with tumours of all categories.