Intravascular lymphomatosis is characterized by a proliferation of malignant lymphoid cells within the lumen of arteriola, capillaria or venula. It is a very rare neoplasia (less than 200 cases reported). Many organs can be involved, but preferentially the central nervous system. The diagnosis is sometimes performed only by a post mortem study. We report a case of a 74 year-old man with biological markers of inflammation, progressive dyspnea and myalgia. A muscular biopsy revealed the diagnosis.