Two cases of intravascular lymphoma (IVL) were diagnosed by endoscopic biopsy. Both patients were admitted to our hospital with a fever of an unknown origin. An elevated serum level of soluble interleukin-2 receptor antibody suggested IVL. An upper gastrointestinal endoscopy was performed. A biopsy of both the reddened and normal gastroduodenal mucosa (Case 1) and a biopsy of a gastric antral ulcer, multiple polyploid lesions resembling submucosal tumors in the duodenum, and the patient's normal mucosa (Case 2) revealed vascular infiltration by CD20-positive atypical lymphocytes, confirming the diagnosis of IVL. The performance of a gastrointestinal biopsy for suspected IVL is important, even if there are no visible endoscopic abnormalities.