Hyperprolactinemia is associated with endothelial dysfunction and atherogenic risk factors, but carotid intima media thickness (IMT) has not been studied in hyperprolactinemic patients. To determine whether untreated hyperprolactinemia contributes to increased carotid IMT. Thirty-one prolactinoma patients and 60 healthy controls were respectively studied. Participants underwent hormone evaluation. Anthropometric parameters (body mass index and blood pressure), inflammatory markers (high-sensitivity C-reactive protein and fibrinogen), serum glucose, insulin, lipid and apolipoprotein profiles were also determined. Endothelial function measured as the flow-mediated dilation (FMD) of a brachial artery and carotid IMT were evaluated using high-resolution ultrasonography. Multivariate linear regression analysis was applied to identify independent determinants of FMD and carotid IMT. Triglycerides, homeostasis model assessment of insulin resistance, apolipoprotein (apo)B/apoA-I ratio, high-sensitivity C-reactive protein (hsCRP) and fibrinogen were significantly higher, while apoA-I was significantly lower in patients with prolactinomas than in the controls. Meanwhile, decreased FMD and increased carotid IMT were observed in hyperprolactinemic group. Serum prolactin was positively correlated with triglycerides, apoB/apoA-I ratio, hypogonadal, hsCRP and fibrinogen (P < 0.05), but inversely associated with apoA-I and HDL-C (P ≤ 0.001). Moreover, prolactin was found negatively correlated with FMD (r = -0.576, P < 0.0001), and positively correlated with mean carotid IMT (r = 0.652, P < 0.0001). Multivariate regression analysis revealed that prolactin determined, independent of traditional risk factors, FMD (B = -0.589, 95% confidence interval (CI) -2.525 to -0.804, P = 0.001) and mean carotid IMT (B = 0.527, 95% CI 0.027-0.069, P < 0.0001). Hyperprolactinemia may be involved in the preclinical increase in carotid IMT, directly or by promoting atherogenic factors, including insulin resistance, low-grade inflammation and endothelial dysfunction. Additional studies are warranted to confirm our findings and explore the mechanisms underlying prolactin-associated early atherosclerosis.