['Although epipodophyllotoxins are commonly used in contemporary treatment regimens for acute lymphoblastic leukemia (ALL), their potential role in CNS-directed therapy has received little attention. We prospectively studied 20 children during initial remission of ALL and 16 children at relapse to assess CSF penetration of etoposide.', 'Simultaneous plasma and CSF concentrations were assessed at a median of 2.8 hours (range, 0.4 to 5.3) after an intravenous (i.v.) or oral dose in 41 paired samples.', 'Etoposide given at 300 mg/m2 i.v. to patients during first remission and at 50 or 25 mg/m2 orally to those in relapse resulted in median CSF levels of 0.175 mumol/L (range, .066 to 2.12), 0.011 mumol/L (range, .004 to .032), and 0.007 mumol/L (range, .003 to .014), respectively. The CSF etoposide concentration was > or = 10 nmol/L in 20 of 20, five of 10, and two of 11 courses following 300 mg/m2 i.v., 50 mg/m2 orally, and 25 mg/m2 orally, respectively, and was positively related to both the concurrent etoposide plasma concentration (R2 = .64) and to dose (R2 = .73). The median ratio of CSF to plasma concentration was 0.30% (range, 0.09% to 3.12%), which was not related to dose, plasma concentration, or time postdose at which samples were obtained, but was positively correlated with the CSF protein concentration (R2 = 0.43, P = .006). Both the absolute etoposide CSF concentrations (P = .008) and the ratio of CSF to plasma concentrations (P = .023) were higher among first-remission patients who had CSF leukemic blasts at diagnosis compared with those without CSF blasts.'] Does etoposide achieve potentially cytotoxic concentrations in CSF of children with acute lymphoblastic leukemia?