We investigate aged garlic extract (AGE) consumption on plasma total nitric oxide synthase (NOS) activity and nitric oxide (NO) levels in patients with atherosclerosis. Eleven patients with severe atherosclerosis and 15 control subjects participated in the study. In addition to their daily diets and routine medical therapy, patients consumed AGE (1 ml/kg body weight; approximately 0.2 g dry garlic/kg body weight) for 6 months. Blood samples were obtained before and after AGE consumption. Additionally, blood samples were obtained from 15 healthy subjects. Total NOS activity and NO level were measured in the plasma samples of the groups. Plasma NOS activity was significantly lowered but NO levels were elevated in the atherosclerotic patients as compared with the controls. AGE consumption resulted in significant increases in plasma NOS activity and NO level of the patients. We conclude that AGE consumption increases plasma NOS activity in atherosclerotic patients. This result suggests that AGE may arise from its NOS-inducing and NO-producing activities. Lowered NOS activity observed in the atherosclerotic patients indicates decreased NO production. However, the higher NO levels observed in the atherosclerotic patients might arise from NO-producing medicines, like isosorbide dinitrate, consumed during the study period.