A 40-year-old white male developed Mycoplasma pneumoniae pneumonia (IgM titer 1:256) as well as autoimmune hemolytic anemia due to cold agglutinins (titer of 1:512). Four days after admission to the hospital, he developed an acute superior mesenteric artery (SMA) thrombosis. Four feet of ischemic small bowel were resected. A follow-up angiogram again showed SMA thrombosis and a left popliteal artery thrombosis. The patient was returned to the operating room and underwent thrombectomy of the affected arteries. The following day, he again developed a left popliteal artery thrombosis requiring thrombectomy. Plasmapheresis, Coumadin and prednisone were implemented. No further thrombotic events occurred. Hypercoagulability workup was negative. Pathology samples revealed vasculitis. Based on a negative hypercoagulability workup, nonrecurrence of thrombotic events after treatment, and in the absence of any structural abnormalities of the affected arteries, an autoimmune phenomenon with damage to the endothelium was thought to have played a role in the mechanism of thrombosis.