Ultraviolet (UV) skin exposure was measured in a group of 21 marathon runners during the Barcelona and Madrid marathons. This was accomplished by means of a personal dosimeter attached to the runner's chest or back. At the same time, ambient UV irradiance and heat stress index were also measured by means of a spectroradiometer and a heat versus exertion monitor, respectively. The latter is an analog of the wet bulb globe temperature. The results showed that during the races the marathoners received a mean radiation of 10 and 4.5 of the minimal erythematous dose. In spite of this high UVB dose the runners did not show unwanted acute skin responses such as sunburn. Since marathon runners can be at high risk for skin cancer, the guidelines for protection of the skin from the American Cancer Society against UV radiation are highly recommended for this group of athletes.