Magnetic Resonance Imaging (MRI) is considered a useful procedure for evaluating the intracranial diseases. Leukemic infiltration in either the oculomotor or trigeminal nerve is rarely seen by MRI. We herein describe the diagnostic assessment and outcome of an adult patient suffering from acute lymphoblastic leukemia (L2, FAB). An 18-year-old female was suspected to have ALL. She complained of nausea, vomiting, and double vision for a month, and further examination was required. Fiber gastroscope examination revealed no abnormal lesions in the upper gastrointestinal tract. Furthermore, a plain brain CT indicated no abnormalities in the brain. However, examination of a Gd-DTPA enhanced MRI and the liquor revealed that she had leptomeningeal leukemia with oculomotor and trigeminal nerve infiltration. Systemic administration of methotrexate (MTX) and cytarabine, and intrathecal injection of MTX were partially effective. Repeated Gd-DTPA enhanced MRI showed the shrinkage and reduced enhancement of oculomotor and trigeminal nerve, parallel to the improvement in the liquor. This case demonstrates the importance of MRI not only in the evaluation of therapy, but also in the early diagnosis of central nervous system leukemia.