A study reported in today’s BMJ confirms that multiple sclerosis is probably an epigenetic disease with in utero exposure to low vitamin D levels determining disease susceptibility in later life. Judith Staples and colleagues show that low maternal exposure to ultraviolet radiation in the first trimester of pregnancy is independently associated with subsequent risk of MS in their offspring in Australia. The adjusted incidence rate ratio for MS is 1.32 (95% confidence interval 1.10 to 1.58, P<0.01) for those subjects born in November or December compared with those born in May or June. As expected the inverse is observed in the Northern hemisphere. How exposure to low vitamin D levels in utero determines disease susceptibility is unknown but is clearly an area of major scientific interest and is being actively pursued by our group. The public health implications of these observations are profound and support the need for high-dose vitamin D supplementation in early pregnancy. In a collaborative study between Queen Mary University of London and Oxford University we are exploring this phenomenon further and would urge anyone with MS to complete our online survey.