Introduction: One of the cardiovascular effects of hyperthyroidism is increased carotid intima media thickness (CIMT). The aim of this study is to investigate the CIMT in patients with Graves' hyperthyroidism and the effect of propylthiouracil (PTU) therapy on CIMT. Method: Twenty-six patients with Graves' hyperthyroidism and 33 healthy controls were included in the study. CIMT was measured at the right and left external carotid arteries in every patient in both groups. CIMT was measured before and after the PTU therapy in patients with Graves' hyperthyroidism. Results: There was a significant difference in CIMT between the group of Graves' hyperthyroid patients and the control group (0.72 versus 0.55 mm, P < 0.0001) at baseline. Twenty-five of 26 patients with Graves' disease were followed up for 18 months prospectively. Euthyroidism has been achieved in 21 patients. After 18 months of treatment, CIMT decreased significantly compared with the baseline values [0.84 (0.54-1.3) to 0.72 (0.50-1.2), change 0.12 mm, P < 0.001]. Conclusion: Graves' hyperthyroidism is associated with atherosclerosis as assessed by CIMT. Treatment of Graves' hyperthyroidism with PTU decreases the CIMT.