Forty-two patients with localized carcinoma of the prostate have been treated with interstitial implantation of iodine 125 seeds after a thorough staging process. The procedure was well tolerated and carried acceptable morbidity and no mortality. Of major concern, however, was the failure of local control of the tumour in a high proportion (88%) of patients at 1 year. Combined interstitial and external beam radiotherapy may overcome this.