The extent of and factors controlling arsenic penetration into the central nervous system (CNS) remain unclear. Elemental arsenic levels in 67 paired cerebrospinal fluid (CSF) and plasma samples from 9 patients with acute promyelocytic leukemia (APL) on oral arsenic trioxide (As2O3), obtained during intrathecal chemotherapy (treatment of CNS APL, n = 6; prophylaxis, n = 3) were measured. Median arsenic levels of CSF and plasma were 95.8 nmol/L (range, 3.5-318.9 nmol/L) and 498.9 nmol/L (range, 36.3-1892.8 nmol/L). As a group, CSF and plasma arsenic was linearly correlated (P < .001), with CSF at 17.7% the plasma level. The CSF/plasma arsenic ratio, which reflected the arsenic CSF penetration efficiency, varied significantly in individual patients (P < .001). Repeated intrathecal chemotherapy and presence of blasts in CSF did not affect the CSF/plasma arsenic ratio. Plasma arsenic was the only significant determinant of CSF arsenic levels. CSF arsenic was present at therapeutically meaningful levels, implying that As2O3 therapy might be beneficial in CNS APL.