To determine if geoclimatic factors may influence the nature and frequency of dermatomyositis (DM), polymyositis, and associated autoantibodies around the world.

We assessed, in the first global evaluation of these conditions, the relationship between 13 geoclimatic variables that may modulate disease and the relative proportion of DM and its associated autoantibody anti–Mi‐2, directed against an SNF2‐superfamily helicase associated with the nucleosome remodeling and histone acetylation and deacetylation complex, in a global myositis population. Altogether, 919 consecutive patients from populations at 15 locations were studied.

Univariate and multivariate analyses demonstrated that of the variables evaluated, surface ultraviolet (UV) radiation intensity (irradiance) most strongly contributed to the relative proportion of DM and was strongly related to the proportion of anti–Mi‐2 autoantibodies (weighted r = 0.939, P < 4 × 10^‐7 and weighted r = 0.69, P = 0.02, respectively). Published ethnogeographic immunogenetic allele frequencies imply that the striking differences in the proportion of DM‐ and DM‐specific autoantibodies observed around the world are not the result of inherent global variations in known genetic risk factors.

These data suggest that UV radiation exposure may modulate the clinical and immunologic expression of an autoimmune disease in different populations around the world.