Two patients with neoplastic disease had transfusion-induced malaria. In a patient with acute myelogenous leukemia infected with Plasmodium vivax, neither his underlying disease nor intensive cytotoxic chemotherapy appeared to ameliorate or worsen the clinical course of his infection. In a splenectomized patient with metastatic carcinoma of the colon, P malariae infection was associated with a fulminant course simulating cerebral malaria. Despite delay in diagnosis, both patients responded dramatically to antimalarial chemotherapy and both developed appreciable antibody responses.