We conducted a case-control study to evaluate the relationship between ischemic stroke in young adults (<45 years of age) and plasma homocysteine (Hcy), plasma folate and vitamin B(12), after a methionine load. We studied 42 patients with a history of ischemic stroke and 29 controls with a negative clinical history of cardio- or cerebrovascular diseases, venous thrombosis and renal disease. A fasting blood sample was drawn from each participant; the second and third samples were collected, respectively, 120 and 240 min after the methionine load. Whilst there was no difference between controls and patients in basal total homocysteine (tHcy), we found a statistically significant difference in both the 120- and 240-min samples. We compared the basal and 240-min tHcy in patients and controls. We obtained a median value of 17.8 and 11.6 micromol/l in patients and controls, respectively. The difference between these two values was highly significant. The methionine loading test (MLT) reveals Hcy metabolism abnormalities that were not revealed by the basal sample. MLT may help identify and treat this new risk factor, which seems to be both atherogenic and prothrombotic, and is hypothesized to operate through various mechanisms.