Information on primary tumour size, status of the pelvic lymph nodes, histological type and macroscopic tumour appearance, as well as age and sex, was available at presentation for 394 patients in the Co-operative Urological Cancer Group's prospective randomised trial for T3 cancer of the urinary bladder. An apparently significant prognostic effect of age and sex was shown to be entirely consistent with the effect of natural mortality. Primary tumour size was found to be the single most powerful prognostic factor (P = 0.002), followed by nodal status (P = 0.02). These factors do not act independently. Multivariate analysis showed that 75% of the effect of all the six variables and their first order interactions could be explained by a single prognostic grouping based on tumour size and nodal status only. Three levels for this grouping are proposed: node-negative small tumour, node-negative moderate tumour and either node-positive or large tumour. The 3-year survival probabilities for the three prognostic groups were 85.7% (95% CI 57.2 and 96.4%), 60.3% (48.0 and 71.5%) and 33.3% (23.5 and 44.8%) respectively.