A 24-year-old woman underwent craniotomy for falx meningioma (5 cm in diameter) on October 24, 1995. The deepest part of the tumor was located in the anterior horn of the lateral ventricle, which was not resected. The histology was clear cell meningioma (CCM), aggressive in nature. The MIB-1 labeling index was high (11%). She underwent gamma knife (GK) radiosurgery for the residual tumor with an irradiation dose of 16 Gy at the tumor periphery on May 24, 1996. The postradiosurgical course was uneventful. The residual intraventricular tumor gradually decreased in size, but the peripheral portion gradually grew into the diencephalic region. The patient remained in good condition for 5 years until September 2001, when she exhibited memory disturbance and lethargy. Magnetic resonance imaging demonstrated a large tumor (4.5 cm in diameter) in the diencephalon, compressing the optic nerves and fornix. The calculated tumor doubling time was 120 days. A second craniotomy was performed on October 9, 2001. The tumor was totally resected through the anterior transcallosal approach. The histology and the MIB-1 labeling index of the tissue from the second operation did not differ markedly from those of the first operation. Neither tumor recurrence nor metastasis has been observed to date. GK radiosurgery contributed to control of the residual intraventricular tumor, but the peripheral portion of the tumor, which received a relatively low radiation dose (16 Gy), grew rapidly. This suggests that a marginal dose of 16 Gy may not be sufficient for control of CCM.