To determine the percentage of patients who developed multiple central nervous system (CNS) gliomas during postoperative radiation therapy and chemotherapy, the authors reviewed the records of 1047 patients treated between December 2, 1976, and August 16, 1985, who had an original diagnosis of supratentorial glioblastoma multiforme or other anaplastic glioma. The occurrence of multiple lesions was verified by neurodiagnostic studies (computerized tomography or myelography) or by findings at operation or autopsy. Twelve patients (1.1%) who presented with multiple lesions were excluded from this analysis. There were 405 patients with glioblastoma multiforme; their median age was 46.5 years (range 22 to 70 years). Eighteen (5%) of these patients had multiple CNS lesions, five of which were in the spinal cord. The median time from diagnosis to detection of the second lesion in this group was 59.5 weeks (range 10 to 182 weeks). There were 630 patients with anaplastic glioma (which included mixed malignant glioma and highly anaplastic, gemistocytic, moderately anaplastic, and anaplastic astrocytomas); their median age was 30 years (range 2 to 62 years). Fifty-four (8.6%) of these patients had multiple lesions, 10 of which were in the spinal cord; only one case of extraneural metastasis was found. The median time from diagnosis to detection of the second lesion in this group was 101 weeks (range 14 to 459 weeks). These results show that more than 90% of CNS gliomas recur at the site of the original tumor. Considering the high frequency of intellectual dysfunction after whole-brain radiation therapy, the use of focal radiation fields appears to be the most judicious approach to the treatment of patients with gliomas.