Experience gained in the treatment of carcinoma of the urinary bladder with radioactive Tantalum is reported. The tumours were classified on the lines laid down by the Union Internationale Contre le Cancer (1963). The Tantalum needles used and their application are described briefly. Emphasis is placed on the advantages of the method over other forms of interstitial radiation. A review of earlier studies is followed by a report on the results of treatment in 62 patients under observation for up to nine years. The tumours were exclusively malignant carcinomata, 13 differentiated and 49 anaplastic transitional epithelial carcinomata, of which one third were at stage P3. The three-and five-year survival rates were 40 and 31 per cent, respectively. Radiation reactions were few and recorded only during the early stages of the period of treatment. Comparison of the results of this treatment with those of resection without interstitial radiation is attempted. The conclusion is drawn that there is little evidence...