Evaluation of: Mowry EM, Krupp LB, Milazzo M et al. Vitamin D status is associated with relapse rate in pediatric-onset multiple sclerosis. Ann. Neurol. 67, 618-624 (2010). In 1960, Sir Donald Acheson suggested a link between multiple sclerosis and solar radiation. Some 50 years and numerous studies later, there is still no certainty about the role of UV or vitamin D, the sun-induced hormone, on the course of multiple sclerosis. In order to further clarify this issue, the article by Mowry et al. assesses the level of vitamin D in patients at the time of multiple sclerosis onset and correlates it with the number of subsequent clinical relapses. The conclusion is that every 10 ng/ml increase in the adjusted vitamin D level is associated with a 34% decrease in the rate of relapse. This result reinforces the rationale for a randomized double-blind placebo-controlled clinical trial. Several criteria for a well-designed trial are proposed.