This paper reports clinicopathological findings concerning an enlarged bulky cyst and the tumor cavity following local administration of an anticancer agent combined with radiotherapy in two patients suffering from malignant glioma. Case 1: This 69 year-old man who had been diagnosed as having glioblastoma in the right parietal lobe had received local chemotherapy after the first operation. Simultaneously radiotherapy of 69 Gy in total dose was performed. At the second operation for the tumor, cyst formation was clinically confirmed and necrotomy as well as evacuation of the large cyst was performed after adjuvant therapy. The patient died at a time ten months after the first surgical operation. Case 2: This is the case of a 48 year-old man who was diagnosed as having gemistocytic astrocytoma in the left frontal lobe. The first surgical operation was performed and was followed by local chemotherapy as well as radiotherapy (total dose of 90 Gy in two sessions). The second surgical operation of the recurrent tumor, with necrotomy and evacuation of the large cyst were performed after adjuvant therapy. The patient expired at a time sixty-five months after the first surgical operation. Relevant to the chemotherapy, adriamycin (ADM) (0.5mg) and methotrexate (MTX) (1mg) were administered through the Ommaya's reservoir into the tumor bed at craniotomy. The usual doses of ADM and MTX amounted to 5.0mg respectively. Through conventional CT and MRI, formation of a cyst including abundant membranous debris or septi was identified in both cases.(ABSTRACT TRUNCATED AT 250 WORDS)