We present a retrospective cost-effectiveness analysis using data from a randomised controlled trial (EORTC 22863) of the addition of early hormonal therapy with a luteinising hormone-releasing hormone (LHRH) analogue to radiotherapy in the treatment of patients with locally advanced prostate cancer. Data on the use of medical resources were extracted from the hospital charts of 90 patients recruited into the trial by one French hospital. Costs are assessed from the viewpoint of the French healthcare financing system and adjusted for censoring. Expected costs per patient of each treatment is related to the expected outcome, mean survival time, estimated by a restricted means analysis. The time point of restriction is determined by statistical criteria. In the base case analysis with a cut-off time point at 8.58 years, the combined therapy group (COMB) had a gain in mean survival time of 1.06 years (7.05 versus 5.99 years) and a reduction of average total costs of 12700 French francs (FF) (58300 FF versus 71000 FF). The analysis of uncertainty uses bootstrap techniques with 5000 replicates to examine the joint distribution of cost and survival outcomes. In 76% of the cases, COMB results in longer mean survival time and lower costs than the radiotherapy group (RT). In cases where COMB therapy raises costs (13% of the cases), it is rarely by more than 20000 FF per patient, no matter the size of the associated survival gain. It is thus highly likely that COMB should be considered a cost-effective option compared with RT for these patients. The exact result of the economic evaluation is decisively determined by the restriction time point selected for the determination of mean survival time, partly also because the average total costs of the two treatments develop entirely differently as a function of the survival time.