Rheumatoid arthritis (RA) is a chronic inflammatory joint disease whose main complication is accelerated atheroma responsible for high rates of cardiovascular morbidity and mortality. Hyperhomocysteinemia is among the factors incriminated in RA-associated atheroma. We managed a 46-year-old patient with RA who required admission to evaluate severe arterial and venous disease with involvement of the coronary, renal, and peripheral arteries. She had no laboratory evidence of rheumatoid vasculitis or conventional cardiovascular risk factors (diabetes and hypercholesterolemia) and had never smoked. Her serum homocysteine level was elevated to 20.9 micromol/L as a result of a homozygous C667T mutation in the methylenetetrahydrofolate (MTHFR) gene. Folate and vitamin B12 levels were normal. A circulating anticoagulant was identified. Hyperhomocysteinemia, which is defined as a homocysteine level greater than 15 micromol/L, is a risk factor for arterial and venous disease. Hyperhomocysteinemia is found in 20%-42% of patients with RA. Methotrexate therapy is the most common causative factor. Other causes include MTHFR deficiency, vitamin B12 deficiency, renal failure, old age, and smoking. Whatever the underlying cause, folic acid supplementation returns the homocysteine level to normal.