Lymphomatoid granulomatosis (LYG) is a systemic granulomatous disease characterized by B-cell proliferation of uncertain malignant potential. It most frequently affects the lungs but also occasionally affects the central nervous system. Its pathophysiology is unclear in numerous respects, thus making it difficult to diagnose and treat. We recently encountered a case of LYG that was followed clinically and histologically for 14 months. A 55-year-old man was hospitalized with multiple brain tumors for which the final diagnosis was not made until the second surgery, 14 months after the first intervention. Following the diagnosis, he was treated with steroid pulse therapy. At present, 3.5 years after the onset of LYG, the patient is in good condition with no signs of tumor recurrence. Although LYG is usually graded on the basis of histological findings, this patient showed no histological changes or any increase in disease grade during the 14-month follow-up period.