This study is all over the net right now but I thought I would post anyways. I have had a close call with this one, so I always like to make sure people are on top of this.
If you have white/fair skin and do a lot of marathon running you should be aware that your chances of developing melanoma might be higher, say researchers from the Medical University of Graz, Austria. Dermatologists there found that a noticeable number of their melanoma patients were ultra-marathon runners.
(Melanoma = Malignant tumor of the skin. Skin cancer)
In fact, two of the authors of this study were enthusiastic marathon runners and wondered whether their activity might be linked to an increased risk of developing skin cancer.
Link.