A case of malignant lymphoma presented with various neuropsychological symptoms which made it difficult to achieve the diagnosis and therapy. A 9-year-old boy was referred to our hospital with complaints of psychological symptoms, impairment of consciousness and generalized convulsive seizures. A lumbar puncture revealed infiltration of numerous blasts which were positive for immature B cell markers. CT scan showed a mass in the right ethmoidal sinus, but no space occupying lesion in the brain. The tumor cells in the right ethmoidal sinus had the same phenotypes as those in the CSF. Thus a diagnosis of non-Hodgkin's lymphoma, developing in the right ethmoidal sinus and infiltrating into the central nervous system, was established. The blast cells in the liquor and the tumor of the right ethmoidal sinus disappeared after induction of systemic chemotherapy, irradiation and intrathecal chemotherapy. However, intracranial bleeding occurred during the chemotherapy. He died of respiratory failure 10 months after the start of treatment.