A 56-year-old man was admitted to our hospital with leukocytosis, anemia, and thrombocytopenia. Acute monoblastic leukemia was diagnosed. Two subsequent courses of consolidation chemotherapy consisted of conventional doses of cytarabine and intermediate-dose cytarabine. Intermediate-dose cytarabine was infused intravenously every 12 hr for 6 days. On day 15 after the final infusion of cytarabine, the patient suffered headache, and on day 21, he experienced a decrease in sensation on the sole of his left foot. Magnetic resonance imaging (MRI) of the brain revealed widespread areas of white matter edema. Cerebrospinal fluid (CSF) examination revealed an increase in the number of cells to 31 mm(-3); the majority were lymphocytes. No infiltration of leukemia cells was seen. After 2 months, brain MRI findings were normal. The clinicoradiologic features of the case were consistent with reversible posterior leukoencephalopathy syndrome (RPLS). RPLS in the present case was unlikely to have been caused by direct neurotoxicity because (1) the doses of cytarabine (500 mg/m(2); total dose 9.2 g) were much smaller than those in reported cases and were repeatedly infused until RPLS developed; (2) the RPLS developed 21 days after the final infusion of cytarabine, a much longer period than previously reported; (3) the slight leukocytosis in the CSF observed on day 33 might also have been related to the cellular immune responses evoked by the infused cytarabine. These details suggest not only that direct cerebral neurotoxicity of cytarabine but also that some type of allergic response may have been involved in the development of RPLS.