An association between low serum vitamin D levels and poorer melanoma survival has been reported. We have studied inheritance of a polymorphism of the GC gene, rs2282679, coding for the vitamin D-binding protein, which is associated with lower serum levels of vitamin D, in a meta-analysis of 3137 melanoma patients. The aim was to investigate evidence for a causal relationship between vitamin D and outcome (Mendelian randomization). The variant was not associated with reduced OS in the UK cohort, per-allele hazard ratio (HR) for death 1.23 (95% CI 0.93,1.64. In the smaller cohorts, HR in OS analysis was 1.07 (95% CI 0.88,1.3) and for all cohorts combined, HR for OS was 1.09 (95% CI 0.93,1.29). There was evidence of increased melanoma specific deaths in the 7 cohorts for which these data were available. The lack of unequivocal findings despite the large sample size illustrates the difficulties of implementing Mendelian randomization. This article is protected by copyright. All rights reserved.