In the field of urological oncology there can be no greater therapeutic challenge for the urologist and radiotherapist than a bladder tumour which is found to be infiltrating muscle. The threat to life by such lesions is of varying magnitude, depending upon the precise depth of mural infiltration. In patients with tumour limited to the superficial muscle (stage T2), cure is anticipated, whereas in those cases showing involvement of deep muscle or perivesical fat (stage T3a and T3b), therapeutic failure leading to death within two years is not unexpected.Bladder tumours with muscle involvement are not early enough to be dismissed after one or more sessions of transurethral cystodiathermy, nor so late as to be relegated to palliative procedures. A radical approach is required, although in some cases not necessarily amounting to total cystectomy nor to large volume external beam therapy.T2 tumours Before the megavoltage era, interstitial irradiation was an attractive method of treatment, enabling high doses to be delivered locally in a deep-seated organ without heavily irradiating the normal surrounding tissues. With the advent of megavoltage equipment and with more aggressive methods of endoscopic resection, the indications for interstitial treatment for bladder cancer have diminished. Thus, at the Christie Hospital, Manchester, the proportion of cases treated by radioactive implant has fallen from 41% in 1953in , to 3% in 1969in (Pointon 1973.Interstitial irradiation in the bladder now appears to be used at only a few centres, mainly in Europe. The techniques and materials for this type of treatment have been reviewed elsewhere (Bloom & Wallace 1971).The ideal case for interstitial therapy is one with a solitary tumour of relatively low-grade malignancy whose base does not exceed 4-5 cm in diameter, with early muscle infiltration and with no obvious malignant or premalignant changes in the bladder away from the tumour site. This technique has also been employed for patients with mucosal tumours (T1) which are large, sessile and solid-looking, and especially when biopsy shows poor differentiation and there is doubt regarding the precise depth of infiltration.The principal advantages associated with interstitial irradiation are that a substantial cure rate is obtained with a low operative morbidity and mortality (2-4%); that irradiation is restricted to a portion of the bladder wall, with sparing of the extravesical tissues; and that if cure is not achieved, other methods of treatment are not precluded.