To investigate the effect of highly purified eicosapentaenoic acid (EPA) on the progression of diabetic macroangiopathy, we performed an open-label randomized prospective trial. A total of 81 Japanese type 2 diabetes were randomly assigned to the EPA (1800mg/day) treated group or the control group. Carotid intima-media thickness (IMT) and brachial-ankle pulse wave velocity (baPWV) were evaluated before and after treatment in both groups. Sixty patients (EPA group, n=30; control group, n=30) completed this study. During the study period of 2.1+/-0.2 years, the mean IMT and max IMT of the EPA treated group showed a significant annual decrease compared with that of the control group (mean IMT, -0.029+/-0.112mm versus 0.016+/-0.109mm, respectively, P=0.029; max IMT, -0.084+/-0.113mm versus -0.005+/-0.108mm, respectively, P=0.0008). The baPWV was also improved significantly in the EPA treated group compared with the control group (-22.1+/-127.9cm/s versus 62.3+/-223cm/s, respectively, P=0.021). Multiple regression analysis showed that the administration of EPA was a significant and independent factor associated with an annual improvement of mean IMT (R(2)=0.067). In summary, this is the first demonstration that administration of purified EPA improves the carotid IMT and the baPWV in patients with type 2 diabetes.