Twelve consecutive patients with carcinoma in situ (CIS) and 17 with T1G3 transitional cell cancer of the bladder were given intravesical Bacillus Calmette-Guérin (BCG) therapy after transurethral resection of all visible lesions. BCG (150 mg in 50 ml normal saline) was given once weekly for 12 weeks and then once monthly for 6 months. The median follow-up was 2.5 years in the CIS group and 3 years in the T1G3 group, with cystoscopy, cytology and random biopsies performed every 3 months. Two of the 12 CIS tumours relapsed, with stage progression (T2G3). Of the 17 T1G3 transitional cell cancers, two relapsed with Ta-1, G1-2 and four progressed to T2-3G3 requiring cystectomy (3) or irradiation (1). One of these four patients died of metastatic disease. BCG therapy thus was effective in preventing progression CIS, but less useful for such purpose in T1G3 transitional cell tumours, for which it cannot be recommended as standard treatment.