The authors report their experience with interstitial brachytherapy in 46 patients with malignant brain tumors. Twenty-three patients received implants after external radiation for newly diagnosed malignant astrocytoma, as part of a randomized study (Group I). Eighteen patients received implants for recurrent malignant astrocytoma (Group II) and 3 for recurrent solitary cerebral metastasis from adenocarcinoma of the lung (Group III). Two additional patients received implants as part of their initial therapy for a radiation-induced malignant astrocytoma and a malignant fibrous histiocytoma. In all patients high-activity iodine-125 sources were implanted temporarily by a stereotactic technique. To date 50 patients have been entered in the randomized study with a maximum follow-up of 31 months. The small numbers and short follow-up preclude any conclusions regarding the efficacy of brachytherapy used as part of initial therapy. The median survival to date in the 23 patients in this group who received implants is 60 weeks. The median survival to date for the 18 patients who received implants for recurrent malignant astrocytoma is 44 weeks postimplant. Two of the three patients who received implants for recurrent solitary metastasis are alive at 41 and 49 weeks postimplant; a third died at 39 weeks postimplant. Significant complications attributable to interstitial brachytherapy have been observed in 10 of 46 patients (21.7%). A second operation for clinical and radiological deterioration was performed in 12 patients (26.1%) 8 to 105 weeks after brachytherapy. The authors conclude that interstitial brachytherapy is beneficial for selected patients with recurrent malignant astrocytoma and solitary recurrent metastasis. The role for this therapy as part of the initial management of patients with malignant astrocytoma remains to be adequately determined in ongoing randomized studies.