Introduction: Chloride hot springs have been suggested to have a high thermal retention effect, with various beneficial effects on health. However, the efficacies of such hot springs for preventing arteriosclerosis and improving blood flow have not been scientifically verified. We investigated flow-mediated vasodilation (FMD) as an index of early arteriosclerosis before and after hot spring bathing to study the effects of chloride hot spring baths on the blood vessels. Subjects and methods: The subjects were 17 healthy adult men and women with an average age of 63 years. The Qua I THERME natrium chloride hot spring in Maebashi City, Gunma prefecture, Japan, was used. Measurements were conducted before and 1 h after bathing. Results and discussion: The mean FMD values in all subjects increased significantly 1 h after in comparison to the values before bathing. The FMD values increased in women, but not in men. Body Mass Index (BMI) was 25 or higher in five cases, among which one cases showed a decrease in FMD% after bathing. Hot spring therapy was shown to increase nitric oxide (NO) levels and expand the vascular smooth muscles temporarily. The results also indicated inhibition of platelet agglutination, smooth muscle propagation, leukocyte adhesion and active oxygen production accompanying bathing, which may lead to improvement of vascular endothelial function.