To conduct a prospective randomized study to evaluate cilostazol, a phosphodiesterase 3 inhibitor, and compare it with aspirin for the prevention of the progression of atherosclerosis in patients with type 2 diabetes (T2D). Materials and methods: Fifty patients with T2D and carotid atherosclerotic plaques were randomly assigned to either a 200 mg/d cilostazol (CTZ) group or a 100 mg/d aspirin (ASA) group for 6 months. The primary endpoint was change in plaque volume measured by carotid three-dimensional ultrasonography. The secondary endpoints were changes in carotid intima-media thickness (IMT) and endothelial function, assessed by laser Doppler. Results: Twenty-four patients in the CTZ group and 23 in the ASA group were included in the final analysis. The mean ± SD age of male (n = 20) and female (n = 16) patients was 62.2 and 59.1 years, respectively. The total plaque volume was slightly decreased in the CTZ group (from 183.8 ± 52.5 to 181.5 ± 54.0 mm 3 ; P = .567), but significantly increased in the ASA group (from 112.9 ± 21.2 to 128.5 ± 23.3 mm 3 ; P = .043). A significant regression in the maximum IMT was observed only in the CTZ group (right: from 2.19 ± 0.17 to 1.96 ± 0.12 mm; left: from 2.02 ± 0.20 to 1.72 ± 0.19 mm). The CTZ group exhibited an increase in HDL cholesterol and a decrease in triglycerides and liver enzymes. Conclusions: Cilostazol treatment for 6 months significantly attenuated the progression of carotid plaque compared with aspirin in patients with T2D (NCT03248401). K E Y W O R D S carotid 3D ultrasonography, carotid atherosclerosis, cilostazol, type 2 diabetes 1 | INTRODUCTION Diabetes is a well-known risk factor for cardiovascular disease (CVD), such as coronary artery disease and stroke. The risk of stroke is twoto threefold higher in patients with type 2 diabetes (T2D) compared with the general population. 1 In patients with T2D, the prevalence of ischaemic stroke is higher than that of haemorrhagic stroke compared with those without diabetes. 2 Furthermore, it has been suggested that patients with T2D without previous myocardial infarction have an equal risk of myocardial infarction to that of non-diabetic patients with previous myocardial infarction. 3 Therefore, inhibiting the progression of atherosclerosis to prevent CVD is important in patients Dong-Hwa Lee and Eun Ju Chun contributed to this study equally.