ABSTRACTA boy with acute lymphoblastic leukemia showed cerebrospinal fluid and blood eosinophilia and basophilia. No cause for the eosinophilia or basophilia was identified, but they preceded the appearance of leukemic lymphoblasts in both CSF and blood. The first appearance of basophils and eosinophils in the CSF was associated with acute symptomatology and responded completely to antileukemic therapy. Though treatment with intrathecal drugs reversed major neurological signs, there was evidence of generalized cerebral cortical atrophy. The possible mechanisms responsible for this unusual cellular reaction include: 1) reaction to antileukemic therapy; 2) immunological reaction to the lymphoblasts; 3) ectopic hormone production; 4) common stem cell origin of lymphoblasts, basophils, and eosinophils. The last explanation seems the most likely. This is a report of a boy with acute lymphoblastic leukemia (ALL) who developed central nervous system (CNS) and bone marrow relapse of leukemia following 2 years of remission. He showed marked eosinophilia and basophilia in the cerebrospinal fluid (CSF) associated with blood eosinophilia and basophilia. Only Budka et al.(1)have reported a similar patient with acute leukemia and CSF eosinophilia and basophilia; their patient showed no blood eosinophilia or basophilia. The gradual accumulation of case reports of eosinophilic or basophilic meningitis with Hodgkin's or non-Hodgkin's lymphoma(2,5)suggests that this finding is of some importance.