Gliomas of the CNS associated with tuberous sclerosis have been well documented; malignant degeneration to glioblastoma multiforme, however, is rare. We studied a 17-year-old boy with stigmata of tuberous sclerosis and a cerebral glioblastoma multiforme. The rarity of this occurrence suggests that neoplasms arising from hamartomas may behave differently than those CNS tumors that arise apparently de novo.