BACKGROUND After completing basic research on the anti-tumor effects and neurotoxicity of 5-fluoro-2'-deoxyuridine (FdUrd) and the balance of thymidine kinase and thymidine phosphorylase activities and confirming the safety of intrathecal FdUrd administration in a previous clinical study of meningeal carcinomatosis, intracavitary administration of FdUrd was performed as a second trial in patients with malignant glioma and metastatic brain tumors. METHODS The study population consisted of 13 patients, six with glioblastoma, one with anaplastic astrocytoma and six with metastatic brain tumors. This treatment was applied for cystic, small-volume residual or recurrent tumors. FdUrd (1-10 microg) was administered every day at least 25 times through an Ommaya device placed in the cyst or closed postoperative cavity reconstructed with a patch of galea aponeurotica. Intracavitary chemotherapy with FdUrd was preceded by radiation therapy in two patients but no other adjuvant therapy was performed. RESULTS No side effects such as headache, nuchal pain, convulsive attack, bone marrow suppression or liver dysfunction were observed during the course of chemotherapy. Seven of the 13 patients showed responses: complete response six, minor response one, no change two and progressive disease four after the twenty-fifth intracavitary administration of FdUrd when tumor sizes on CT scans and MRI before and after intracavitary chemotherapy were compared. CONCLUSIONS Intracavitary FdUrd chemotherapy may be useful for the treatment of small-volume tumors.