A 14-year-old boy, suffering from acute lymphoblastic leukemia with meningeal involvement, was treated with intraventricular methotrexate and cytosine arabinoside, administered via an Ommaya reservoir (OR). Three months later, right occipital headache, vomiting, and lethargy appeared. Cerebrospinal fluid specimens showed increased proteins and a right frontal slow-wave focus was evident on the EEG recording. The computed tomography scan revealed white matter hypodensity within the right frontal and rolandic regions. After injection of medium contrast, an abscesslike hyperdensity appeared, surrounding both a well-placed cannula tip and the right frontal horn of the lateral ventricle. Brain swelling and shift signs were also evident. Nine cases of focal methotrexate leukoencephalopathy have been previously reported, and in six of these there was a misplaced OR cannula tip. The focal methotrexate leukoencephalopathy seems to be related to the neurotoxicity of the drugs administered, and may also exist with a well-placed OR cannula tip. Immediate removal of the catheter may be associated with a benign evolution.