Idiopathic guttate hypomelanosis: idiopathic or ultraviolet induced?

Idiopathic guttate hypomelanosis (IGH) is a common but often unrecognized skin disease with obscure etiopathogenesis. As the lesions mostly develop on the exposed parts of the extremities, ultraviolet has been hypothesized to induce these lesions. However, previous statistical studies did not confirm any association between ultraviolet and IGH. We describe a 72‐year‐old woman with mycosis fungoides in whom widespread IGH occurred during narrow band ultraviolet B (UVB) therapy. The lesion development in the trunk following narrow band UVB therapy suggests that these lesions may not be idiopathic and they may be associated with UV exposure although controversy exist in the literature. This case is particular as it reveals strong evidence on this relationship.