The net benefit of BCG immunotherapeutic prophylactic effect on recurrence of superficial bladder tumours was investigated. The BCG treatment group consisted of 121 stage Ta, T1 patients, while the control group, 49 patients, was treated only with transurethral resection. During 3-year follow-up recurrence rate in the control group was 55.1%, while in the BCG group in the two-and-a-half-year follow-up it was 23.9%. The yearly repeated 6-week cycles resulted in decrease of recurrence rate to 11.8%. The recurrence indexes were 2.2 and 0.6 in the control group and in the repeated treatment group respectively. A progression rate of 18.8% was recorded in the control group and 4.1% in the treatment group. Finally both clinical and investigative results were summarised and the necessity of repeated immunotherapeutic BCG courses were emphasized.