BACKGROUND Intravascular large B-cell lymphoma (IVLBCL) is a rare type of extranodal large B-cell lymphoma characterized by involvement of lymphoma cells in the lumina of small blood vessels in various organs. Although many studies have claimed that timely diagnosis and early initiation of chemotherapy are important, the literature on the successful treatment of IVLBCL in elderly patients over the age of 80 years is scarce. CASE REPORT An 82-year-old man presented with abnormal elevation of lactate dehydrogenase at 1605 IU/L and altered mental status, which manifested as forgetfulness, abnormal behavior, and worsening performance status for 2 days. There was no evidence of lymphadenopathy or skin lesions, but a computed tomography scan of the chest showed bilateral diffuse ground-glass opacities. With a primary consideration of IVLBCL, random skin biopsy and transbronchial lung biopsy were performed. Immediately after the pathologic findings on hematoxylin-eosin staining alone revealed abnormal large cells in the small vessels, chemotherapy with cyclophosphamide, doxorubicin, vincristine, and prednisolone (CHOP) at half the usual dose was started; later on, immunostaining revealed CD20 expression in the abnormal lymphoid cells. Immediately after CHOP administration, his mental and physical symptoms rapidly resolved. After 8 cycles of dose-reduced CHOP with rituximab, he remained free from disease progression 2 years after the diagnosis. CONCLUSIONS We presented a case of successful early chemotherapy for IVLBCL in an elderly patient with poor performance status. Early chemotherapy with CHOP, even before obtaining a definitive diagnosis based on immunostaining, may contribute to good outcomes.