The association of Waldenström disease and glioblastoma was observed in two patients. Both diseases were diagnosed almost simultaneously. In the first patient the diagnosis of glioblastoma was made on cerebral biopsy. In the second case, the diagnosis was confirmed at autopsy. The association of Waldenström's disease and glioblastoma is rare but it does not seem to be coincidental. The physiopathological problems related to this coexistence are discussed.